The following sections describe the standard types that are built into the interpreter.
Note
Historically (until release 2.2), Python's built-in types have differed from user-defined types because it was not possible to use the built-in types as the basis for object-oriented inheritance. This limitation no longer exists.
The principal built-in types are numerics, sequences, mappings, files, classes, instances and exceptions.
Some operations are supported by several object types; in particular,
practically all objects can be compared, tested for truth value, and converted
to a string (with the repr() function or the slightly different
str()
function). The latter function is implicitly used when an object is
written by the print()
function.
Any object can be tested for truth value, for use in an if
or
while
condition or as operand of the Boolean operations below. The
following values are considered false:
None
False
- zero of any numeric type, for example,
0
,0L
,0.0
,0j
. - any empty sequence, for example,
''
,()
,[]
. - any empty mapping, for example,
{}
. - instances of user-defined classes, if the class defines a
__nonzero__()
or__len__()
method, when that method returns the integer zero orbool
valueFalse
. 1
All other values are considered true --- so objects of many types are always true.
Operations and built-in functions that have a Boolean result always return 0
or False
for false and 1
or True
for true, unless otherwise stated.
(Important exception: the Boolean operations or
and and
always return
one of their operands.)
and
, or
, not
These are the Boolean operations, ordered by ascending priority:
Operation | Result | Notes |
---|---|---|
x or y | if x is false, then y, else x | (1) |
x and y | if x is false, then x, else y | (2) |
not x | if x is false, then True ,
else False | (3) |
Notes:
- This is a short-circuit operator, so it only evaluates the second argument if the first one is false.
- This is a short-circuit operator, so it only evaluates the second argument if the first one is true.
not
has a lower priority than non-Boolean operators, sonot a == b
is interpreted asnot (a == b)
, anda == not b
is a syntax error.
Comparison operations are supported by all objects. They all have the same
priority (which is higher than that of the Boolean operations). Comparisons can
be chained arbitrarily; for example, x < y <= z
is equivalent to x < y and
y <= z
, except that y is evaluated only once (but in both cases z is not
evaluated at all when x < y
is found to be false).
This table summarizes the comparison operations:
Operation | Meaning | Notes |
---|---|---|
< | strictly less than | |
<= | less than or equal | |
> | strictly greater than | |
>= | greater than or equal | |
== | equal | |
!= | not equal | (1) |
is | object identity | |
is not | negated object identity |
Notes:
!=
can also be written<>
, but this is an obsolete usage kept for backwards compatibility only. New code should always use!=
.
Objects of different types, except different numeric types and different string
types, never compare equal; such objects are ordered consistently but
arbitrarily (so that sorting a heterogeneous array yields a consistent result).
Furthermore, some types (for example, file objects) support only a degenerate
notion of comparison where any two objects of that type are unequal. Again,
such objects are ordered arbitrarily but consistently. The <
, <=
, >
and >=
operators will raise a TypeError
exception when any operand is
a complex number.
Non-identical instances of a class normally compare as non-equal unless the
class defines the __eq__()
method or the __cmp__()
method.
Instances of a class cannot be ordered with respect to other instances of the
same class, or other types of object, unless the class defines either enough of
the rich comparison methods (__lt__()
, __le__()
, __gt__()
, and
__ge__()
) or the __cmp__()
method.
CPython implementation detail: Objects of different types except numbers are ordered by their type names; objects of the same types that don't support proper comparison are ordered by their address.
Two more operations with the same syntactic priority, in
and not in
, are
supported only by sequence types (below).
int
, float
, long
, complex
There are four distinct numeric types: plain integers, long
integers, floating point numbers, and complex numbers. In
addition, Booleans are a subtype of plain integers. Plain integers (also just
called integers) are implemented using long
in C, which gives
them at least 32 bits of precision (sys.maxint
is always set to the maximum
plain integer value for the current platform, the minimum value is
-sys.maxint - 1
). Long integers have unlimited precision. Floating point
numbers are usually implemented using double
in C; information about
the precision and internal representation of floating point numbers for the
machine on which your program is running is available in
sys.float_info
. Complex numbers have a real and imaginary part, which
are each a floating point number. To extract these parts from a complex number
z, use z.real
and z.imag
. (The standard library includes additional
numeric types, fractions
that hold rationals, and decimal
that
hold floating-point numbers with user-definable precision.)
Numbers are created by numeric literals or as the result of built-in functions
and operators. Unadorned integer literals (including binary, hex, and octal
numbers) yield plain integers unless the value they denote is too large to be
represented as a plain integer, in which case they yield a long integer.
Integer literals with an 'L'
or 'l'
suffix yield long integers ('L'
is preferred because 1l
looks too much like eleven!). Numeric literals
containing a decimal point or an exponent sign yield floating point numbers.
Appending 'j'
or 'J'
to a numeric literal yields an imaginary number
(a complex number with a zero real part) which you can add to an integer or
float to get a complex number with real and imaginary parts.
Python fully supports mixed arithmetic: when a binary arithmetic operator has
operands of different numeric types, the operand with the "narrower" type is
widened to that of the other, where plain integer is narrower than long integer
is narrower than floating point is narrower than complex. Comparisons between
numbers of mixed type use the same rule. 2 The constructors int()
,
long()
, float()
, and complex()
can be used to produce numbers
of a specific type.
All built-in numeric types support the following operations. See The power operator and later sections for the operators' priorities.
Operation | Result | Notes |
---|---|---|
x + y | sum of x and y | |
x - y | difference of x and y | |
x * y | product of x and y | |
x / y | quotient of x and y | (1) |
x // y | (floored) quotient of x and y | (4)(5) |
x % y | remainder of x / y | (4) |
-x | x negated | |
+x | x unchanged | |
abs(x) | absolute value or magnitude of x | (3) |
int(x) | x converted to integer | (2) |
long(x) | x converted to long integer | (2) |
float(x) | x converted to floating point | (6) |
complex(re,im) | a complex number with real part re, imaginary part im. im defaults to zero. | |
c.conjugate() | conjugate of the complex number c. (Identity on real numbers) | |
divmod(x, y) | the pair (x // y, x % y) | (3)(4) |
pow(x, y) | x to the power y | (3)(7) |
x ** y | x to the power y | (7) |
Notes:
For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value.
Conversion from floats using
int()
orlong()
truncates toward zero like the related function,math.trunc()
. Use the functionmath.floor()
to round downward andmath.ceil()
to round upward.- See Built-in Functions for a full description.
- Also referred to as integer division. The resultant value is a whole integer, though the result's type is not necessarily int.
float also accepts the strings "nan" and "inf" with an optional prefix "+" or "-" for Not a Number (NaN) and positive or negative infinity.
New in version 2.6.
- Python defines
pow(0, 0)
and0 ** 0
to be1
, as is common for programming languages.
All numbers.Real
types (int
, long
, and
float
) also include the following operations:
Operation | Result |
---|---|
math.trunc(x) | x truncated to Integral |
round(x[,
n]) | x rounded to n digits, rounding ties away from zero. If n is omitted, it defaults to 0. |
math.floor(x) | the greatest integer as a float <= x |
math.ceil(x) | the least integer as a float >= x |
Bitwise operations only make sense for integers. Negative numbers are treated as their 2's complement value (this assumes a sufficiently large number of bits that no overflow occurs during the operation).
The priorities of the binary bitwise operations are all lower than the numeric
operations and higher than the comparisons; the unary operation ~
has the
same priority as the other unary numeric operations (+
and -
).
This table lists the bitwise operations sorted in ascending priority:
Operation | Result | Notes |
---|---|---|
x | y | bitwise or of x and y | |
x ^ y | bitwise exclusive or of x and y | |
x & y | bitwise and of x and y | |
x << n | x shifted left by n bits | (1)(2) |
x >> n | x shifted right by n bits | (1)(3) |
~x | the bits of x inverted |
Notes:
- Negative shift counts are illegal and cause a
ValueError
to be raised. - A left shift by n bits is equivalent to multiplication by
pow(2, n)
. A long integer is returned if the result exceeds the range of plain integers. - A right shift by n bits is equivalent to division by
pow(2, n)
.
The integer types implement the numbers.Integral
abstract base
class. In addition, they provide one more method:
int.bit_length()
long.bit_length()
Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros:
>>> n = -37
>>> bin(n)
'-0b100101'
>>> n.bit_length()
6
More precisely, if x
is nonzero, then x.bit_length()
is the
unique positive integer k
such that 2**(k-1) <= abs(x) < 2**k
.
Equivalently, when abs(x)
is small enough to have a correctly
rounded logarithm, then k = 1 + int(log(abs(x), 2))
.
If x
is zero, then x.bit_length()
returns 0
.
Equivalent to:
def bit_length(self):
s = bin(self) # binary representation: bin(-37) --> '-0b100101'
s = s.lstrip('-0b') # remove leading zeros and minus sign
return len(s) # len('100101') --> 6
New in version 2.7.
The float type implements the numbers.Real
abstract base
class. float also has the following additional methods.
float.as_integer_ratio()
Return a pair of integers whose ratio is exactly equal to the
original float and with a positive denominator. Raises
OverflowError
on infinities and a ValueError
on
NaNs.
New in version 2.6.
float.is_integer()
Return True
if the float instance is finite with integral
value, and False
otherwise:
>>> (-2.0).is_integer()
True
>>> (3.2).is_integer()
False
New in version 2.6.
Two methods support conversion to and from hexadecimal strings. Since Python's floats are stored internally as binary numbers, converting a float to or from a decimal string usually involves a small rounding error. In contrast, hexadecimal strings allow exact representation and specification of floating-point numbers. This can be useful when debugging, and in numerical work.
float.hex()
Return a representation of a floating-point number as a hexadecimal
string. For finite floating-point numbers, this representation
will always include a leading 0x
and a trailing p
and
exponent.
New in version 2.6.
float.fromhex(s)
Class method to return the float represented by a hexadecimal string s. The string s may have leading and trailing whitespace.
New in version 2.6.
Note that float.hex()
is an instance method, while
float.fromhex()
is a class method.
A hexadecimal string takes the form:
[sign] ['0x'] integer ['.' fraction] ['p' exponent]
where the optional sign
may by either +
or -
, integer
and fraction
are strings of hexadecimal digits, and exponent
is a decimal integer with an optional leading sign. Case is not
significant, and there must be at least one hexadecimal digit in
either the integer or the fraction. This syntax is similar to the
syntax specified in section 6.4.4.2 of the C99 standard, and also to
the syntax used in Java 1.5 onwards. In particular, the output of
float.hex()
is usable as a hexadecimal floating-point literal in
C or Java code, and hexadecimal strings produced by C's %a
format
character or Java's Double.toHexString
are accepted by
float.fromhex()
.
Note that the exponent is written in decimal rather than hexadecimal,
and that it gives the power of 2 by which to multiply the coefficient.
For example, the hexadecimal string 0x3.a7p10
represents the
floating-point number (3 + 10./16 + 7./16**2) * 2.0**10
, or
3740.0
:
>>> float.fromhex('0x3.a7p10')
3740.0
Applying the reverse conversion to 3740.0
gives a different
hexadecimal string representing the same number:
>>> float.hex(3740.0)
'0x1.d380000000000p+11'
New in version 2.2.
Python supports a concept of iteration over containers. This is implemented using two distinct methods; these are used to allow user-defined classes to support iteration. Sequences, described below in more detail, always support the iteration methods.
One method needs to be defined for container objects to provide iteration support:
container.__iter__()
Return an iterator object. The object is required to support the iterator
protocol described below. If a container supports different types of
iteration, additional methods can be provided to specifically request
iterators for those iteration types. (An example of an object supporting
multiple forms of iteration would be a tree structure which supports both
breadth-first and depth-first traversal.) This method corresponds to the
tp_iter
slot of the type structure for Python objects in the Python/C
API.
The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:
iterator.__iter__()
Return the iterator object itself. This is required to allow both containers
and iterators to be used with the for
and in
statements.
This method corresponds to the tp_iter
slot of the type structure for
Python objects in the Python/C API.
iterator.next()
Return the next item from the container. If there are no further items, raise
the StopIteration
exception. This method corresponds to the
tp_iternext
slot of the type structure for Python objects in the
Python/C API.
Python defines several iterator objects to support iteration over general and specific sequence types, dictionaries, and other more specialized forms. The specific types are not important beyond their implementation of the iterator protocol.
The intention of the protocol is that once an iterator's next()
method
raises StopIteration
, it will continue to do so on subsequent calls.
Implementations that do not obey this property are deemed broken. (This
constraint was added in Python 2.3; in Python 2.2, various iterators are broken
according to this rule.)
Python's generators provide a convenient way to implement the iterator
protocol. If a container object's __iter__()
method is implemented as a
generator, it will automatically return an iterator object (technically, a
generator object) supplying the __iter__()
and
next()
methods. More information about generators can be found
in the documentation for the yield expression.
str
, unicode
, list
, tuple
, bytearray
, buffer
, xrange
There are seven sequence types: strings, Unicode strings, lists, tuples, bytearrays, buffers, and xrange objects.
For other containers see the built in dict
and set
classes,
and the collections
module.
String literals are written in single or double quotes: 'xyzzy'
,
"frobozz"
. See String literals for more about string literals.
Unicode strings are much like strings, but are specified in the syntax
using a preceding 'u'
character: u'abc'
, u"def"
. In addition
to the functionality described here, there are also string-specific
methods described in the String Methods section. Lists are
constructed with square brackets, separating items with commas: [a, b, c]
.
Tuples are constructed by the comma operator (not within square
brackets), with or without enclosing parentheses, but an empty tuple
must have the enclosing parentheses, such as a, b, c
or ()
. A
single item tuple must have a trailing comma, such as (d,)
.
Bytearray objects are created with the built-in function bytearray()
.
Buffer objects are not directly supported by Python syntax, but can be created
by calling the built-in function buffer()
. They don't support
concatenation or repetition.
Objects of type xrange are similar to buffers in that there is no specific syntax to
create them, but they are created using the xrange()
function. They don't
support slicing, concatenation or repetition, and using in
, not in
,
min()
or max()
on them is inefficient.
Most sequence types support the following operations. The in
and not in
operations have the same priorities as the comparison operations. The +
and
*
operations have the same priority as the corresponding numeric operations.
3 Additional methods are provided for Mutable Sequence Types.
This table lists the sequence operations sorted in ascending priority. In the table, s and t are sequences of the same type; n, i and j are integers:
Operation | Result | Notes |
---|---|---|
x in s | True if an item of s is
equal to x, else False | (1) |
x not in s | False if an item of s is
equal to x, else True | (1) |
s + t | the concatenation of s and t | (6) |
s * n, n * s | equivalent to adding s to itself n times | (2) |
s[i] | ith item of s, origin 0 | (3) |
s[i:j] | slice of s from i to j | (3)(4) |
s[i:j:k] | slice of s from i to j with step k | (3)(5) |
len(s) | length of s | |
min(s) | smallest item of s | |
max(s) | largest item of s | |
s.index(x) | index of the first occurrence of x in s | |
s.count(x) | total number of occurrences of x in s |
Sequence types also support comparisons. In particular, tuples and lists are compared lexicographically by comparing corresponding elements. This means that to compare equal, every element must compare equal and the two sequences must be of the same type and have the same length. (For full details see Comparisons in the language reference.)
Notes:
- When s is a string or Unicode string object the
in
andnot in
operations act like a substring test. In Python versions before 2.3, x had to be a string of length 1. In Python 2.3 and beyond, x may be a string of any length. Values of n less than
[UNKNOWN NODE doctest_block]0
are treated as0
(which yields an empty sequence of the same type as s). Note that items in the sequence s are not copied; they are referenced multiple times. This often haunts new Python programmers; consider:What has happened is that
[UNKNOWN NODE doctest_block][[]]
is a one-element list containing an empty list, so all three elements of[[]] * 3
are references to this single empty list. Modifying any of the elements oflists
modifies this single list. You can create a list of different lists this way:Further explanation is available in the FAQ entry How do I create a multidimensional list?.
- If i or j is negative, the index is relative to the end of sequence s:
len(s) + i
orlen(s) + j
is substituted. But note that-0
is still0
. - The slice of s from i to j is defined as the sequence of items with index
k such that
i <= k < j
. If i or j is greater thanlen(s)
, uselen(s)
. If i is omitted orNone
, use0
. If j is omitted orNone
, uselen(s)
. If i is greater than or equal to j, the slice is empty. - The slice of s from i to j with step k is defined as the sequence of
items with index
x = i + n*k
such that0 <= n < (j-i)/k
. In other words, the indices arei
,i+k
,i+2*k
,i+3*k
and so on, stopping when j is reached (but never including j). When k is positive, i and j are reduced tolen(s)
if they are greater. When k is negative, i and j are reduced tolen(s) - 1
if they are greater. If i or j are omitted orNone
, they become "end" values (which end depends on the sign of k). Note, k cannot be zero. If k isNone
, it is treated like1
. CPython implementation detail: If s and t are both strings, some Python implementations such as CPython can usually perform an in-place optimization for assignments of the form
s = s + t
ors += t
. When applicable, this optimization makes quadratic run-time much less likely. This optimization is both version and implementation dependent. For performance sensitive code, it is preferable to use thestr.join()
method which assures consistent linear concatenation performance across versions and implementations.Changed in version 2.4: Formerly, string concatenation never occurred in-place.
Below are listed the string methods which both 8-bit strings and
Unicode objects support. Some of them are also available on bytearray
objects.
In addition, Python's strings support the sequence type methods
described in the Sequence Types --- str, unicode, list, tuple, bytearray, buffer, xrange section. To output formatted strings
use template strings or the %
operator described in the
String Formatting Operations section. Also, see the re
module for
string functions based on regular expressions.
str.capitalize()
Return a copy of the string with its first character capitalized and the rest lowercased.
For 8-bit strings, this method is locale-dependent.
str.center(width[, fillchar])
Return centered in a string of length width. Padding is done using the specified fillchar (default is a space).
Changed in version 2.4: Support for the fillchar argument.
str.count(sub[, start[, end]])
Return the number of non-overlapping occurrences of substring sub in the range [start, end]. Optional arguments start and end are interpreted as in slice notation.
str.decode([encoding[, errors]])
Decodes the string using the codec registered for encoding. encoding
defaults to the default string encoding. errors may be given to set a
different error handling scheme. The default is 'strict'
, meaning that
encoding errors raise UnicodeError
. Other possible values are
'ignore'
, 'replace'
and any other name registered via
codecs.register_error()
, see section Codec Base Classes.
New in version 2.2.
Changed in version 2.3: Support for other error handling schemes added.
Changed in version 2.7: Support for keyword arguments added.
str.encode([encoding[, errors]])
Return an encoded version of the string. Default encoding is the current
default string encoding. errors may be given to set a different error
handling scheme. The default for errors is 'strict'
, meaning that
encoding errors raise a UnicodeError
. Other possible values are
'ignore'
, 'replace'
, 'xmlcharrefreplace'
, 'backslashreplace'
and
any other name registered via codecs.register_error()
, see section
Codec Base Classes. For a list of possible encodings, see section
Standard Encodings.
New in version 2.0.
Changed in version 2.3: Support for 'xmlcharrefreplace'
and 'backslashreplace'
and other error
handling schemes added.
Changed in version 2.7: Support for keyword arguments added.
str.endswith(suffix[, start[, end]])
Return True
if the string ends with the specified suffix, otherwise return
False
. suffix can also be a tuple of suffixes to look for. With optional
start, test beginning at that position. With optional end, stop comparing
at that position.
Changed in version 2.5: Accept tuples as suffix.
str.expandtabs([tabsize])
Return a copy of the string where all tab characters are replaced by one or
more spaces, depending on the current column and the given tab size. Tab
positions occur every tabsize characters (default is 8, giving tab
positions at columns 0, 8, 16 and so on). To expand the string, the current
column is set to zero and the string is examined character by character. If
the character is a tab (\t
), one or more space characters are inserted
in the result until the current column is equal to the next tab position.
(The tab character itself is not copied.) If the character is a newline
(\n
) or return (\r
), it is copied and the current column is reset to
zero. Any other character is copied unchanged and the current column is
incremented by one regardless of how the character is represented when
printed.
str.find(sub[, start[, end]])
Return the lowest index in the string where substring sub is found within
the slice s[start:end]
. Optional arguments start and end are
interpreted as in slice notation. Return -1
if sub is not found.
str.format(*args, **kwargs)
Perform a string formatting operation. The string on which this method is
called can contain literal text or replacement fields delimited by braces
{}
. Each replacement field contains either the numeric index of a
positional argument, or the name of a keyword argument. Returns a copy of
the string where each replacement field is replaced with the string value of
the corresponding argument.
See Format String Syntax for a description of the various formatting options that can be specified in format strings.
This method of string formatting is the new standard in Python 3, and
should be preferred to the %
formatting described in
String Formatting Operations in new code.
New in version 2.6.
str.index(sub[, start[, end]])
Like find()
, but raise ValueError
when the substring is not found.
str.isalnum()
Return true if all characters in the string are alphanumeric and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.isalpha()
Return true if all characters in the string are alphabetic and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.isdigit()
Return true if all characters in the string are digits and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.islower()
Return true if all cased characters 4 in the string are lowercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.isspace()
Return true if there are only whitespace characters in the string and there is at least one character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.istitle()
Return true if the string is a titlecased string and there is at least one character, for example uppercase characters may only follow uncased characters and lowercase characters only cased ones. Return false otherwise.
For 8-bit strings, this method is locale-dependent.
str.isupper()
Return true if all cased characters 4 in the string are uppercase and there is at least one cased character, false otherwise.
For 8-bit strings, this method is locale-dependent.
str.join(iterable)
Return a string which is the concatenation of the strings in iterable.
A TypeError
will be raised if there are any non-string values in
iterable, including bytes
objects. The separator between
elements is the string providing this method.
str.ljust(width[, fillchar])
Return the string left justified in a string of length width. Padding is done
using the specified fillchar (default is a space). The original string is
returned if width is less than or equal to len(s)
.
Changed in version 2.4: Support for the fillchar argument.
str.lower()
Return a copy of the string with all the cased characters 4 converted to lowercase.
For 8-bit strings, this method is locale-dependent.
str.lstrip([chars])
Return a copy of the string with leading characters removed. The chars
argument is a string specifying the set of characters to be removed. If omitted
or None
, the chars argument defaults to removing whitespace. The chars
argument is not a prefix; rather, all combinations of its values are stripped:
Changed in version 2.2.2: Support for the chars argument.
str.partition(sep)
Split the string at the first occurrence of sep, and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing the string itself, followed by two empty strings.
New in version 2.5.
str.replace(old, new[, count])
Return a copy of the string with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced.
str.rfind(sub[, start[, end]])
Return the highest index in the string where substring sub is found, such
that sub is contained within s[start:end]
. Optional arguments start
and end are interpreted as in slice notation. Return -1
on failure.
str.rindex(sub[, start[, end]])
Like rfind()
but raises ValueError
when the substring sub is not
found.
str.rjust(width[, fillchar])
Return the string right justified in a string of length width. Padding is done
using the specified fillchar (default is a space). The original string is
returned if width is less than or equal to len(s)
.
Changed in version 2.4: Support for the fillchar argument.
str.rpartition(sep)
Split the string at the last occurrence of sep, and return a 3-tuple containing the part before the separator, the separator itself, and the part after the separator. If the separator is not found, return a 3-tuple containing two empty strings, followed by the string itself.
New in version 2.5.
str.rsplit([sep[, maxsplit]])
Return a list of the words in the string, using sep as the delimiter string.
If maxsplit is given, at most maxsplit splits are done, the rightmost
ones. If sep is not specified or None
, any whitespace string is a
separator. Except for splitting from the right, rsplit()
behaves like
split()
which is described in detail below.
New in version 2.4.
str.rstrip([chars])
Return a copy of the string with trailing characters removed. The chars
argument is a string specifying the set of characters to be removed. If omitted
or None
, the chars argument defaults to removing whitespace. The chars
argument is not a suffix; rather, all combinations of its values are stripped:
Changed in version 2.2.2: Support for the chars argument.
str.split([sep[, maxsplit]])
Return a list of the words in the string, using sep as the delimiter
string. If maxsplit is given, at most maxsplit splits are done (thus,
the list will have at most maxsplit+1
elements). If maxsplit is not
specified or -1
, then there is no limit on the number of splits
(all possible splits are made).
If sep is given, consecutive delimiters are not grouped together and are
deemed to delimit empty strings (for example, '1,,2'.split(',')
returns
['1', '', '2']
). The sep argument may consist of multiple characters
(for example, '1<>2<>3'.split('<>')
returns ['1', '2', '3']
).
Splitting an empty string with a specified separator returns ['']
.
If sep is not specified or is None
, a different splitting algorithm is
applied: runs of consecutive whitespace are regarded as a single separator,
and the result will contain no empty strings at the start or end if the
string has leading or trailing whitespace. Consequently, splitting an empty
string or a string consisting of just whitespace with a None
separator
returns []
.
For example, ' 1 2 3 '.split()
returns ['1', '2', '3']
, and
' 1 2 3 '.split(None, 1)
returns ['1', '2 3 ']
.
str.splitlines([keepends])
Return a list of the lines in the string, breaking at line boundaries. This method uses the universal newlines approach to splitting lines. Line breaks are not included in the resulting list unless keepends is given and true.
Python recognizes "\r"
, "\n"
, and "\r\n"
as line boundaries for
8-bit strings.
For example:
>>> 'ab c\n\nde fg\rkl\r\n'.splitlines()
['ab c', '', 'de fg', 'kl']
>>> 'ab c\n\nde fg\rkl\r\n'.splitlines(True)
['ab c\n', '\n', 'de fg\r', 'kl\r\n']
Unlike split()
when a delimiter string sep is given, this
method returns an empty list for the empty string, and a terminal line
break does not result in an extra line:
>>> "".splitlines()
[]
>>> "One line\n".splitlines()
['One line']
For comparison, split('\n')
gives:
>>> ''.split('\n')
['']
>>> 'Two lines\n'.split('\n')
['Two lines', '']
unicode.splitlines([keepends])
Return a list of the lines in the string, like str.splitlines()
.
However, the Unicode method splits on the following line boundaries,
which are a superset of the universal newlines recognized for
8-bit strings.
Representation | Description |
---|---|
\n | Line Feed |
\r | Carriage Return |
\r\n | Carriage Return + Line Feed |
\v or \x0b | Line Tabulation |
\f or \x0c | Form Feed |
\x1c | File Separator |
\x1d | Group Separator |
\x1e | Record Separator |
\x85 | Next Line (C1 Control Code) |
\u2028 | Line Separator |
\u2029 | Paragraph Separator |
Changed in version 2.7: \v
and \f
added to list of line boundaries.
str.startswith(prefix[, start[, end]])
Return True
if string starts with the prefix, otherwise return False
.
prefix can also be a tuple of prefixes to look for. With optional start,
test string beginning at that position. With optional end, stop comparing
string at that position.
Changed in version 2.5: Accept tuples as prefix.
str.strip([chars])
Return a copy of the string with the leading and trailing characters removed.
The chars argument is a string specifying the set of characters to be removed.
If omitted or None
, the chars argument defaults to removing whitespace.
The chars argument is not a prefix or suffix; rather, all combinations of its
values are stripped:
Changed in version 2.2.2: Support for the chars argument.
str.swapcase()
Return a copy of the string with uppercase characters converted to lowercase and vice versa.
For 8-bit strings, this method is locale-dependent.
str.title()
Return a titlecased version of the string where words start with an uppercase character and the remaining characters are lowercase.
The algorithm uses a simple language-independent definition of a word as groups of consecutive letters. The definition works in many contexts but it means that apostrophes in contractions and possessives form word boundaries, which may not be the desired result:
>>> "they're bill's friends from the UK".title()
"They'Re Bill'S Friends From The Uk"
A workaround for apostrophes can be constructed using regular expressions:
>>> import re
>>> def titlecase(s):
... return re.sub(r"[A-Za-z]+('[A-Za-z]+)?",
... lambda mo: mo.group(0)[0].upper() +
... mo.group(0)[1:].lower(),
... s)
...
>>> titlecase("they're bill's friends.")
"They're Bill's Friends."
For 8-bit strings, this method is locale-dependent.
str.translate(table[, deletechars])
Return a copy of the string where all characters occurring in the optional argument deletechars are removed, and the remaining characters have been mapped through the given translation table, which must be a string of length 256.
You can use the maketrans()
helper function in the string
module to create a translation table. For string objects, set the table
argument to None
for translations that only delete characters:
New in version 2.6: Support for a None
table argument.
For Unicode objects, the translate()
method does not accept the optional
deletechars argument. Instead, it returns a copy of the s where all
characters have been mapped through the given translation table which must be a
mapping of Unicode ordinals to Unicode ordinals, Unicode strings or None
.
Unmapped characters are left untouched. Characters mapped to None
are
deleted. Note, a more flexible approach is to create a custom character mapping
codec using the codecs
module (see encodings.cp1251
for an
example).
str.upper()
Return a copy of the string with all the cased characters 4 converted to
uppercase. Note that str.upper().isupper()
might be False
if s
contains uncased characters or if the Unicode category of the resulting
character(s) is not "Lu" (Letter, uppercase), but e.g. "Lt" (Letter, titlecase).
For 8-bit strings, this method is locale-dependent.
str.zfill(width)
Return the numeric string left filled with zeros in a string of length
width. A sign prefix is handled correctly. The original string is
returned if width is less than or equal to len(s)
.
New in version 2.2.2.
The following methods are present only on unicode objects:
unicode.isnumeric()
Return True
if there are only numeric characters in S, False
otherwise. Numeric characters include digit characters, and all characters
that have the Unicode numeric value property, e.g. U+2155,
VULGAR FRACTION ONE FIFTH.
unicode.isdecimal()
Return True
if there are only decimal characters in S, False
otherwise. Decimal characters include digit characters, and all characters
that can be used to form decimal-radix numbers, e.g. U+0660,
ARABIC-INDIC DIGIT ZERO.
String and Unicode objects have one unique built-in operation: the %
operator (modulo). This is also known as the string formatting or
interpolation operator. Given format % values
(where format is a string
or Unicode object), %
conversion specifications in format are replaced
with zero or more elements of values. The effect is similar to the using
sprintf()
in the C language. If format is a Unicode object, or if any
of the objects being converted using the %s
conversion are Unicode objects,
the result will also be a Unicode object.
If format requires a single argument, values may be a single non-tuple object. 5 Otherwise, values must be a tuple with exactly the number of items specified by the format string, or a single mapping object (for example, a dictionary).
A conversion specifier contains two or more characters and has the following components, which must occur in this order:
- The
'%'
character, which marks the start of the specifier. - Mapping key (optional), consisting of a parenthesised sequence of characters
(for example,
(somename)
). - Conversion flags (optional), which affect the result of some conversion types.
- Minimum field width (optional). If specified as an
'*'
(asterisk), the actual width is read from the next element of the tuple in values, and the object to convert comes after the minimum field width and optional precision. - Precision (optional), given as a
'.'
(dot) followed by the precision. If specified as'*'
(an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision. - Length modifier (optional).
- Conversion type.
When the right argument is a dictionary (or other mapping type), then the
formats in the string must include a parenthesised mapping key into that
dictionary inserted immediately after the '%'
character. The mapping key
selects the value to be formatted from the mapping. For example:
In this case no *
specifiers may occur in a format (since they require a
sequential parameter list).
The conversion flag characters are:
Flag | Meaning |
---|---|
'#' | The value conversion will use the "alternate form" (where defined below). |
'0' | The conversion will be zero padded for numeric values. |
'-' | The converted value is left adjusted (overrides the '0'
conversion if both are given). |
' ' | (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion. |
'+' | A sign character ('+' or '-' ) will precede the conversion
(overrides a "space" flag). |
A length modifier (h
, l
, or L
) may be present, but is ignored as it
is not necessary for Python -- so e.g. %ld
is identical to %d
.
The conversion types are:
Conversion | Meaning | Notes |
---|---|---|
'd' | Signed integer decimal. | |
'i' | Signed integer decimal. | |
'o' | Signed octal value. | (1) |
'u' | Obsolete type -- it is identical to 'd' . | (7) |
'x' | Signed hexadecimal (lowercase). | (2) |
'X' | Signed hexadecimal (uppercase). | (2) |
'e' | Floating point exponential format (lowercase). | (3) |
'E' | Floating point exponential format (uppercase). | (3) |
'f' | Floating point decimal format. | (3) |
'F' | Floating point decimal format. | (3) |
'g' | Floating point format. Uses lowercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
'G' | Floating point format. Uses uppercase exponential format if exponent is less than -4 or not less than precision, decimal format otherwise. | (4) |
'c' | Single character (accepts integer or single character string). | |
'r' | String (converts any Python object using repr()). | (5) |
's' | String (converts any Python object using
str() ). | (6) |
'%' | No argument is converted, results in a '%'
character in the result. |
Notes:
- The alternate form causes a leading zero (
'0'
) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero. - The alternate form causes a leading
'0x'
or'0X'
(depending on whether the'x'
or'X'
format was used) to be inserted before the first digit. The alternate form causes the result to always contain a decimal point, even if no digits follow it.
The precision determines the number of digits after the decimal point and defaults to 6.
The alternate form causes the result to always contain a decimal point, and trailing zeroes are not removed as they would otherwise be.
The precision determines the number of significant digits before and after the decimal point and defaults to 6.
The
%r
conversion was added in Python 2.0.The precision determines the maximal number of characters used.
If the object or format provided is a
unicode
string, the resulting string will also beunicode
.The precision determines the maximal number of characters used.
- See PEP 237.
Since Python strings have an explicit length, %s
conversions do not assume
that '\0'
is the end of the string.
Changed in version 2.7: %f
conversions for numbers whose absolute value is over 1e50 are no
longer replaced by %g
conversions.
Additional string operations are defined in standard modules string
and
re
.
The xrange
type is an immutable sequence which is commonly used for
looping. The advantage of the xrange
type is that an xrange
object will always take the same amount of memory, no matter the size of the
range it represents. There are no consistent performance advantages.
XRange objects have very little behavior: they only support indexing, iteration,
and the len()
function.
List and bytearray
objects support additional operations that allow
in-place modification of the object. Other mutable sequence types (when added
to the language) should also support these operations. Strings and tuples
are immutable sequence types: such objects cannot be modified once created.
The following operations are defined on mutable sequence types (where x is
an arbitrary object):
Operation | Result | Notes |
---|---|---|
s[i] = x | item i of s is replaced by x | |
s[i:j] = t | slice of s from i to j is replaced by the contents of the iterable t | |
del s[i:j] | same as s[i:j] = [] | |
s[i:j:k] = t | the elements of s[i:j:k]
are replaced by those of t | (1) |
del s[i:j:k] | removes the elements of
s[i:j:k] from the list | |
s.append(x) | same as s[len(s):len(s)] =
[x] | (2) |
s.extend(t) or
s += t | for the most part the same as
s[len(s):len(s)] = t | (3) |
s *= n | updates s with its contents repeated n times | (11) |
s.count(x) | return number of i's for
which s[i] == x | |
s.index(x[, i[, j]]) | return smallest k such that
s[k] == x and i <= k <
j | (4) |
s.insert(i, x) | same as s[i:i] = [x] | (5) |
s.pop([i]) | same as x = s[i]; del s[i];
return x | (6) |
s.remove(x) | same as del s[s.index(x)] | (4) |
s.reverse() | reverses the items of s in place | (7) |
s.sort([cmp[, key[,
reverse]]]) | sort the items of s in place | (7)(8)(9)(10) |
Notes:
- t must have the same length as the slice it is replacing.
- The C implementation of Python has historically accepted multiple parameters and implicitly joined them into a tuple; this no longer works in Python 2.0. Use of this misfeature has been deprecated since Python 1.4.
- t can be any iterable object.
Raises
ValueError
when x is not found in s. When a negative index is passed as the second or third parameter to theindex()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously,
index()
didn't have arguments for specifying start and stop positions.When a negative index is passed as the first parameter to the
insert()
method, the list length is added, as for slice indices. If it is still negative, it is truncated to zero, as for slice indices.Changed in version 2.3: Previously, all negative indices were truncated to zero.
- The
pop()
method's optional argument i defaults to-1
, so that by default the last item is removed and returned. - The
sort()
andreverse()
methods modify the list in place for economy of space when sorting or reversing a large list. To remind you that they operate by side effect, they don't return the sorted or reversed list. The
sort()
method takes optional arguments for controlling the comparisons.cmp specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument:
cmp=lambda x,y: cmp(x.lower(), y.lower())
. The default value isNone
.key specifies a function of one argument that is used to extract a comparison key from each list element:
key=str.lower
. The default value isNone
.reverse is a boolean value. If set to
True
, then the list elements are sorted as if each comparison were reversed.In general, the key and reverse conversion processes are much faster than specifying an equivalent cmp function. This is because cmp is called multiple times for each list element while key and reverse touch each element only once. Use
functools.cmp_to_key()
to convert an old-style cmp function to a key function.Changed in version 2.3: Support for
None
as an equivalent to omitting cmp was added.Changed in version 2.4: Support for key and reverse was added.
- Starting with Python 2.3, the
sort()
method is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal --- this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade). CPython implementation detail: While a list is being sorted, the effect of attempting to mutate, or even inspect, the list is undefined. The C implementation of Python 2.3 and newer makes the list appear empty for the duration, and raises
ValueError
if it can detect that the list has been mutated during a sort.- The value n is an integer, or an object implementing
__index__()
. Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained fors * n
under Sequence Types --- str, unicode, list, tuple, bytearray, buffer, xrange.
set
, frozenset
A set object is an unordered collection of distinct hashable objects.
Common uses include membership testing, removing duplicates from a sequence, and
computing mathematical operations such as intersection, union, difference, and
symmetric difference.
(For other containers see the built in dict
, list
,
and tuple
classes, and the collections
module.)
New in version 2.4.
Like other collections, sets support x in set
, len(set)
, and for x in
set
. Being an unordered collection, sets do not record element position or
order of insertion. Accordingly, sets do not support indexing, slicing, or
other sequence-like behavior.
There are currently two built-in set types, set
and frozenset
.
The set
type is mutable --- the contents can be changed using methods
like add()
and remove()
. Since it is mutable, it has no
hash value and cannot be used as either a dictionary key or as an element of
another set. The frozenset
type is immutable and hashable ---
its contents cannot be altered after it is created; it can therefore be used as
a dictionary key or as an element of another set.
As of Python 2.7, non-empty sets (not frozensets) can be created by placing a
comma-separated list of elements within braces, for example: {'jack',
'sjoerd'}
, in addition to the set
constructor.
The constructors for both classes work the same:
class set([iterable])
class frozenset([iterable])
Return a new set or frozenset object whose elements are taken from
iterable. The elements of a set must be hashable. To
represent sets of sets, the inner sets must be frozenset
objects. If iterable is not specified, a new empty set is
returned.
Instances of set
and frozenset
provide the following
operations:
len(s)
Return the number of elements in set s (cardinality of s).
x in s
Test x for membership in s.
x not in s
Test x for non-membership in s.
isdisjoint(other)
Return True
if the set has no elements in common with other. Sets are
disjoint if and only if their intersection is the empty set.
New in version 2.6.
issubset(other)
set <= other
Test whether every element in the set is in other.
set < other
Test whether the set is a proper subset of other, that is,
set <= other and set != other
.
issuperset(other)
set >= other
Test whether every element in other is in the set.
set > other
Test whether the set is a proper superset of other, that is, set >=
other and set != other
.
union(*others)
set | other | ...
Return a new set with elements from the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection(*others)
set & other & ...
Return a new set with elements common to the set and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference(*others)
set - other - ...
Return a new set with elements in the set that are not in the others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference(other)
set ^ other
Return a new set with elements in either the set or other but not both.
copy()
Return a new set with a shallow copy of s.
Note, the non-operator versions of union()
, intersection()
,
difference()
, and symmetric_difference()
, issubset()
, and
issuperset()
methods will accept any iterable as an argument. In
contrast, their operator based counterparts require their arguments to be
sets. This precludes error-prone constructions like set('abc') & 'cbs'
in favor of the more readable set('abc').intersection('cbs')
.
Both set
and frozenset
support set to set comparisons. Two
sets are equal if and only if every element of each set is contained in the
other (each is a subset of the other). A set is less than another set if and
only if the first set is a proper subset of the second set (is a subset, but
is not equal). A set is greater than another set if and only if the first set
is a proper superset of the second set (is a superset, but is not equal).
Instances of set
are compared to instances of frozenset
based on their members. For example, set('abc') == frozenset('abc')
returns True
and so does set('abc') in set([frozenset('abc')])
.
The subset and equality comparisons do not generalize to a total ordering
function. For example, any two non-empty disjoint sets are not equal and are not
subsets of each other, so all of the following return False
: a<b
,
a==b
, or a>b
. Accordingly, sets do not implement the __cmp__()
method.
Since sets only define partial ordering (subset relationships), the output of
the list.sort()
method is undefined for lists of sets.
Set elements, like dictionary keys, must be hashable.
Binary operations that mix set
instances with frozenset
return the type of the first operand. For example: frozenset('ab') |
set('bc')
returns an instance of frozenset
.
The following table lists operations available for set
that do not
apply to immutable instances of frozenset
:
update(*others)
set |= other | ...
Update the set, adding elements from all others.
Changed in version 2.6: Accepts multiple input iterables.
intersection_update(*others)
set &= other & ...
Update the set, keeping only elements found in it and all others.
Changed in version 2.6: Accepts multiple input iterables.
difference_update(*others)
set -= other | ...
Update the set, removing elements found in others.
Changed in version 2.6: Accepts multiple input iterables.
symmetric_difference_update(other)
set ^= other
Update the set, keeping only elements found in either set, but not in both.
add(elem)
Add element elem to the set.
remove(elem)
Remove element elem from the set. Raises KeyError
if elem is
not contained in the set.
discard(elem)
Remove element elem from the set if it is present.
pop()
Remove and return an arbitrary element from the set. Raises
KeyError
if the set is empty.
clear()
Remove all elements from the set.
Note, the non-operator versions of the update()
,
intersection_update()
, difference_update()
, and
symmetric_difference_update()
methods will accept any iterable as an
argument.
Note, the elem argument to the __contains__()
, remove()
, and
discard()
methods may be a set. To support searching for an equivalent
frozenset, a temporary one is created from elem.
See also
- Comparison to the built-in set types
- Differences between the
sets
module and the built-in set types.
dict
A mapping object maps hashable values to arbitrary objects.
Mappings are mutable objects. There is currently only one standard mapping
type, the dictionary. (For other containers see the built in
list
, set
, and tuple
classes, and the
collections
module.)
A dictionary's keys are almost arbitrary values. Values that are not
hashable, that is, values containing lists, dictionaries or other
mutable types (that are compared by value rather than by object identity) may
not be used as keys. Numeric types used for keys obey the normal rules for
numeric comparison: if two numbers compare equal (such as 1
and 1.0
)
then they can be used interchangeably to index the same dictionary entry. (Note
however, that since computers store floating-point numbers as approximations it
is usually unwise to use them as dictionary keys.)
Dictionaries can be created by placing a comma-separated list of key: value
pairs within braces, for example: {'jack': 4098, 'sjoerd': 4127}
or {4098:
'jack', 4127: 'sjoerd'}
, or by the dict
constructor.
class dict(**kwarg)
class dict(mapping, **kwarg)
class dict(iterable, **kwarg)
Return a new dictionary initialized from an optional positional argument and a possibly empty set of keyword arguments.
If no positional argument is given, an empty dictionary is created. If a positional argument is given and it is a mapping object, a dictionary is created with the same key-value pairs as the mapping object. Otherwise, the positional argument must be an iterable object. Each item in the iterable must itself be an iterable with exactly two objects. The first object of each item becomes a key in the new dictionary, and the second object the corresponding value. If a key occurs more than once, the last value for that key becomes the corresponding value in the new dictionary.
If keyword arguments are given, the keyword arguments and their values are added to the dictionary created from the positional argument. If a key being added is already present, the value from the keyword argument replaces the value from the positional argument.
To illustrate, the following examples all return a dictionary equal to
{"one": 1, "two": 2, "three": 3}
:
>>> a = dict(one=1, two=2, three=3)
>>> b = {'one': 1, 'two': 2, 'three': 3}
>>> c = dict(zip(['one', 'two', 'three'], [1, 2, 3]))
>>> d = dict([('two', 2), ('one', 1), ('three', 3)])
>>> e = dict({'three': 3, 'one': 1, 'two': 2})
>>> a == b == c == d == e
True
Providing keyword arguments as in the first example only works for keys that are valid Python identifiers. Otherwise, any valid keys can be used.
New in version 2.2.
Changed in version 2.3: Support for building a dictionary from keyword arguments added.
These are the operations that dictionaries support (and therefore, custom mapping types should support too):
len(d)
Return the number of items in the dictionary d.
d[key]
Return the item of d with key key. Raises a KeyError
if key
is not in the map.
If a subclass of dict defines a method __missing__()
and key
is not present, the d[key]
operation calls that method with the key key
as argument. The d[key]
operation then returns or raises whatever is
returned or raised by the __missing__(key)
call.
No other operations or methods invoke __missing__()
. If
__missing__()
is not defined, KeyError
is raised.
__missing__()
must be a method; it cannot be an instance variable:
>>> class Counter(dict):
... def __missing__(self, key):
... return 0
>>> c = Counter()
>>> c['red']
0
>>> c['red'] += 1
>>> c['red']
1
The example above shows part of the implementation of
collections.Counter
. A different __missing__
method is used
by collections.defaultdict
.
New in version 2.5: Recognition of __missing__ methods of dict subclasses.
d[key] = value
Set d[key]
to value.
del d[key]
Remove d[key]
from d. Raises a KeyError
if key is not in the
map.
key in d
Return True
if d has a key key, else False
.
New in version 2.2.
key not in d
Equivalent to not key in d
.
New in version 2.2.
iter(d)
Return an iterator over the keys of the dictionary. This is a shortcut
for iterkeys()
.
clear()
Remove all items from the dictionary.
copy()
Return a shallow copy of the dictionary.
fromkeys(seq[, value])
Create a new dictionary with keys from seq and values set to value.
fromkeys()
is a class method that returns a new dictionary. value
defaults to None
.
New in version 2.3.
get(key[, default])
Return the value for key if key is in the dictionary, else default.
If default is not given, it defaults to None
, so that this method
never raises a KeyError
.
has_key(key)
Test for the presence of key in the dictionary. has_key()
is
deprecated in favor of key in d
.
items()
Return a copy of the dictionary's list of (key, value)
pairs.
CPython implementation detail: Keys and values are listed in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary's history of insertions and deletions.
If items()
, keys()
, values()
, iteritems()
,
iterkeys()
, and itervalues()
are called with no intervening
modifications to the dictionary, the lists will directly correspond. This
allows the creation of (value, key)
pairs using zip()
: pairs =
zip(d.values(), d.keys())
. The same relationship holds for the
iterkeys()
and itervalues()
methods: pairs =
zip(d.itervalues(), d.iterkeys())
provides the same value for
pairs
. Another way to create the same list is pairs = [(v, k) for
(k, v) in d.iteritems()]
.
iteritems()
Return an iterator over the dictionary's (key, value)
pairs. See the
note for dict.items()
.
Using iteritems()
while adding or deleting entries in the dictionary
may raise a RuntimeError
or fail to iterate over all entries.
New in version 2.2.
iterkeys()
Return an iterator over the dictionary's keys. See the note for
dict.items()
.
Using iterkeys()
while adding or deleting entries in the dictionary
may raise a RuntimeError
or fail to iterate over all entries.
New in version 2.2.
itervalues()
Return an iterator over the dictionary's values. See the note for
dict.items()
.
Using itervalues()
while adding or deleting entries in the
dictionary may raise a RuntimeError
or fail to iterate over all
entries.
New in version 2.2.
keys()
Return a copy of the dictionary's list of keys. See the note for
dict.items()
.
pop(key[, default])
If key is in the dictionary, remove it and return its value, else return
default. If default is not given and key is not in the dictionary,
a KeyError
is raised.
New in version 2.3.
popitem()
Remove and return an arbitrary (key, value)
pair from the dictionary.
popitem()
is useful to destructively iterate over a dictionary, as
often used in set algorithms. If the dictionary is empty, calling
popitem()
raises a KeyError
.
setdefault(key[, default])
If key is in the dictionary, return its value. If not, insert key
with a value of default and return default. default defaults to
None
.
update([other])
Update the dictionary with the key/value pairs from other, overwriting
existing keys. Return None
.
update()
accepts either another dictionary object or an iterable of
key/value pairs (as tuples or other iterables of length two). If keyword
arguments are specified, the dictionary is then updated with those
key/value pairs: d.update(red=1, blue=2)
.
Changed in version 2.4: Allowed the argument to be an iterable of key/value pairs and allowed keyword arguments.
values()
Return a copy of the dictionary's list of values. See the note for
dict.items()
.
viewitems()
Return a new view of the dictionary's items ((key, value)
pairs). See
below for documentation of view objects.
New in version 2.7.
viewkeys()
Return a new view of the dictionary's keys. See below for documentation of view objects.
New in version 2.7.
viewvalues()
Return a new view of the dictionary's values. See below for documentation of view objects.
New in version 2.7.
Dictionaries compare equal if and only if they have the same (key,
value)
pairs.
The objects returned by dict.viewkeys()
, dict.viewvalues()
and
dict.viewitems()
are view objects. They provide a dynamic view on the
dictionary's entries, which means that when the dictionary changes, the view
reflects these changes.
Dictionary views can be iterated over to yield their respective data, and support membership tests:
len(dictview)
Return the number of entries in the dictionary.
iter(dictview)
Return an iterator over the keys, values or items (represented as tuples of
(key, value)
) in the dictionary.
Keys and values are iterated over in an arbitrary order which is non-random,
varies across Python implementations, and depends on the dictionary's history
of insertions and deletions. If keys, values and items views are iterated
over with no intervening modifications to the dictionary, the order of items
will directly correspond. This allows the creation of (value, key)
pairs
using zip()
: pairs = zip(d.values(), d.keys())
. Another way to
create the same list is pairs = [(v, k) for (k, v) in d.items()]
.
Iterating views while adding or deleting entries in the dictionary may raise
a RuntimeError
or fail to iterate over all entries.
x in dictview
Return True
if x is in the underlying dictionary's keys, values or
items (in the latter case, x should be a (key, value)
tuple).
Keys views are set-like since their entries are unique and hashable. If all values are hashable, so that (key, value) pairs are unique and hashable, then the items view is also set-like. (Values views are not treated as set-like since the entries are generally not unique.) Then these set operations are available ("other" refers either to another view or a set):
dictview & other
Return the intersection of the dictview and the other object as a new set.
dictview | other
Return the union of the dictview and the other object as a new set.
dictview - other
Return the difference between the dictview and the other object (all elements in dictview that aren't in other) as a new set.
dictview ^ other
Return the symmetric difference (all elements either in dictview or other, but not in both) of the dictview and the other object as a new set.
An example of dictionary view usage:
>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.viewkeys()
>>> values = dishes.viewvalues()
>>> # iteration
>>> n = 0
>>> for val in values:
... n += val
>>> print(n)
504
>>> # keys and values are iterated over in the same order
>>> list(keys)
['eggs', 'bacon', 'sausage', 'spam']
>>> list(values)
[2, 1, 1, 500]
>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> del dishes['sausage']
>>> list(keys)
['spam', 'bacon']
>>> # set operations
>>> keys & {'eggs', 'bacon', 'salad'}
{'bacon'}
File objects are implemented using C's stdio
package and can be
created with the built-in open()
function. File
objects are also returned by some other built-in functions and methods,
such as os.popen()
and os.fdopen()
and the makefile()
method of socket objects. Temporary files can be created using the
tempfile
module, and high-level file operations such as copying,
moving, and deleting files and directories can be achieved with the
shutil
module.
When a file operation fails for an I/O-related reason, the exception
IOError
is raised. This includes situations where the operation is not
defined for some reason, like seek()
on a tty device or writing a file
opened for reading.
Files have the following methods:
file.close()
Close the file. A closed file cannot be read or written any more. Any operation
which requires that the file be open will raise a ValueError
after the
file has been closed. Calling close()
more than once is allowed.
As of Python 2.5, you can avoid having to call this method explicitly if you use
the with
statement. For example, the following code will
automatically close f when the with
block is exited:
from __future__ import with_statement # This isn't required in Python 2.6
with open("hello.txt") as f:
for line in f:
print line,
In older versions of Python, you would have needed to do this to get the same effect:
f = open("hello.txt")
try:
for line in f:
print line,
finally:
f.close()
Note
Not all "file-like" types in Python support use as a context manager for the
with
statement. If your code is intended to work with any file-like
object, you can use the function contextlib.closing()
instead of using
the object directly.
file.flush()
Flush the internal buffer, like stdio
's fflush()
. This may be a
no-op on some file-like objects.
Note
flush()
does not necessarily write the file's data to disk. Use
flush()
followed by os.fsync()
to ensure this behavior.
file.fileno()
Return the integer "file descriptor" that is used by the underlying
implementation to request I/O operations from the operating system. This can be
useful for other, lower level interfaces that use file descriptors, such as the
fcntl
module or os.read()
and friends.
Note
File-like objects which do not have a real file descriptor should not provide this method!
file.isatty()
Return True
if the file is connected to a tty(-like) device, else False
.
Note
If a file-like object is not associated with a real file, this method should not be implemented.
file.next()
A file object is its own iterator, for example iter(f)
returns f (unless
f is closed). When a file is used as an iterator, typically in a
for
loop (for example, for line in f: print line.strip()
), the
next()
method is called repeatedly. This method returns the next input
line, or raises StopIteration
when EOF is hit when the file is open for
reading (behavior is undefined when the file is open for writing). In order to
make a for
loop the most efficient way of looping over the lines of a
file (a very common operation), the next()
method uses a hidden read-ahead
buffer. As a consequence of using a read-ahead buffer, combining next()
with other file methods (like readline()
) does not work right. However,
using seek()
to reposition the file to an absolute position will flush the
read-ahead buffer.
New in version 2.3.
file.read([size])
Read at most size bytes from the file (less if the read hits EOF before
obtaining size bytes). If the size argument is negative or omitted, read
all data until EOF is reached. The bytes are returned as a string object. An
empty string is returned when EOF is encountered immediately. (For certain
files, like ttys, it makes sense to continue reading after an EOF is hit.) Note
that this method may call the underlying C function fread()
more than
once in an effort to acquire as close to size bytes as possible. Also note
that when in non-blocking mode, less data than was requested may be
returned, even if no size parameter was given.
Note
This function is simply a wrapper for the underlying
fread()
C function, and will behave the same in corner cases,
such as whether the EOF value is cached.
file.readline([size])
Read one entire line from the file. A trailing newline character is kept in the string (but may be absent when a file ends with an incomplete line). 6 If the size argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned. When size is not 0, an empty string is returned only when EOF is encountered immediately.
Note
Unlike stdio
's fgets()
, the returned string contains null characters
('\0'
) if they occurred in the input.
file.readlines([sizehint])
Read until EOF using readline()
and return a list containing the lines
thus read. If the optional sizehint argument is present, instead of
reading up to EOF, whole lines totalling approximately sizehint bytes
(possibly after rounding up to an internal buffer size) are read. Objects
implementing a file-like interface may choose to ignore sizehint if it
cannot be implemented, or cannot be implemented efficiently.
file.xreadlines()
This method returns the same thing as iter(f)
.
New in version 2.1.
Deprecated since version 2.3: Use for line in file
instead.
file.seek(offset[, whence])
Set the file's current position, like stdio
's fseek()
. The whence
argument is optional and defaults to os.SEEK_SET
or 0
(absolute file
positioning); other values are os.SEEK_CUR
or 1
(seek relative to the
current position) and os.SEEK_END
or 2
(seek relative to the file's
end). There is no return value.
For example, f.seek(2, os.SEEK_CUR)
advances the position by two and
f.seek(-3, os.SEEK_END)
sets the position to the third to last.
Note that if the file is opened for appending
(mode 'a'
or 'a+'
), any seek()
operations will be undone at the
next write. If the file is only opened for writing in append mode (mode
'a'
), this method is essentially a no-op, but it remains useful for files
opened in append mode with reading enabled (mode 'a+'
). If the file is
opened in text mode (without 'b'
), only offsets returned by tell()
are
legal. Use of other offsets causes undefined behavior.
Note that not all file objects are seekable.
Changed in version 2.6: Passing float values as offset has been deprecated.
file.tell()
Return the file's current position, like stdio
's ftell()
.
Note
On Windows, tell()
can return illegal values (after an fgets()
)
when reading files with Unix-style line-endings. Use binary mode ('rb'
) to
circumvent this problem.
file.truncate([size])
Truncate the file's size. If the optional size argument is present, the file is truncated to (at most) that size. The size defaults to the current position. The current file position is not changed. Note that if a specified size exceeds the file's current size, the result is platform-dependent: possibilities include that the file may remain unchanged, increase to the specified size as if zero-filled, or increase to the specified size with undefined new content. Availability: Windows, many Unix variants.
file.write(str)
Write a string to the file. There is no return value. Due to buffering, the
string may not actually show up in the file until the flush()
or
close()
method is called.
file.writelines(sequence)
Write a sequence of strings to the file. The sequence can be any iterable
object producing strings, typically a list of strings. There is no return value.
(The name is intended to match readlines()
; writelines()
does not
add line separators.)
Files support the iterator protocol. Each iteration returns the same result as
readline()
, and iteration ends when the readline()
method returns
an empty string.
File objects also offer a number of other interesting attributes. These are not required for file-like objects, but should be implemented if they make sense for the particular object.
file.closed
bool indicating the current state of the file object. This is a read-only
attribute; the close()
method changes the value. It may not be available
on all file-like objects.
file.encoding
The encoding that this file uses. When Unicode strings are written to a file,
they will be converted to byte strings using this encoding. In addition, when
the file is connected to a terminal, the attribute gives the encoding that the
terminal is likely to use (that information might be incorrect if the user has
misconfigured the terminal). The attribute is read-only and may not be present
on all file-like objects. It may also be None
, in which case the file uses
the system default encoding for converting Unicode strings.
New in version 2.3.
file.errors
The Unicode error handler used along with the encoding.
New in version 2.6.
file.mode
The I/O mode for the file. If the file was created using the open()
built-in function, this will be the value of the mode parameter. This is a
read-only attribute and may not be present on all file-like objects.
file.name
If the file object was created using open()
, the name of the file.
Otherwise, some string that indicates the source of the file object, of the
form <...>
. This is a read-only attribute and may not be present on all
file-like objects.
file.newlines
If Python was built with universal newlines enabled (the default) this
read-only attribute exists, and for files opened in universal newline read
mode it keeps track of the types of newlines encountered while reading the
file. The values it can take are '\r'
, '\n'
, '\r\n'
, None
(unknown, no newlines read yet) or a tuple containing all the newline types
seen, to indicate that multiple newline conventions were encountered. For
files not opened in universal newlines read mode the value of this attribute
will be None
.
file.softspace
Boolean that indicates whether a space character needs to be printed before
another value when using the print
statement. Classes that are trying
to simulate a file object should also have a writable softspace
attribute, which should be initialized to zero. This will be automatic for most
classes implemented in Python (care may be needed for objects that override
attribute access); types implemented in C will have to provide a writable
softspace
attribute.
New in version 2.7.
memoryview
objects allow Python code to access the internal data
of an object that supports the buffer protocol without copying. Memory
is generally interpreted as simple bytes.
class memoryview(obj)
Create a memoryview
that references obj. obj must support the
buffer protocol. Built-in objects that support the buffer protocol include
str
and bytearray
(but not unicode
).
A memoryview
has the notion of an element, which is the
atomic memory unit handled by the originating object obj. For many
simple types such as str
and bytearray
, an element
is a single byte, but other third-party types may expose larger elements.
len(view)
returns the total number of elements in the memoryview,
view. The itemsize
attribute will give you the
number of bytes in a single element.
A memoryview
supports slicing to expose its data. Taking a single
index will return a single element as a str
object. Full
slicing will result in a subview:
>>> v = memoryview('abcefg')
>>> v[1]
'b'
>>> v[-1]
'g'
>>> v[1:4]
<memory at 0x77ab28>
>>> v[1:4].tobytes()
'bce'
If the object the memoryview is over supports changing its data, the memoryview supports slice assignment:
>>> data = bytearray('abcefg')
>>> v = memoryview(data)
>>> v.readonly
False
>>> v[0] = 'z'
>>> data
bytearray(b'zbcefg')
>>> v[1:4] = '123'
>>> data
bytearray(b'z123fg')
>>> v[2] = 'spam'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: cannot modify size of memoryview object
Notice how the size of the memoryview object cannot be changed.
memoryview
has two methods:
tobytes()
Return the data in the buffer as a bytestring (an object of class
str
).
>>> m = memoryview("abc")
>>> m.tobytes()
'abc'
tolist()
Return the data in the buffer as a list of integers.
>>> memoryview("abc").tolist()
[97, 98, 99]
There are also several readonly attributes available:
format
A string containing the format (in struct
module style) for each
element in the view. This defaults to 'B'
, a simple bytestring.
itemsize
The size in bytes of each element of the memoryview.
shape
A tuple of integers the length of ndim
giving the shape of the
memory as an N-dimensional array.
ndim
An integer indicating how many dimensions of a multi-dimensional array the memory represents.
strides
A tuple of integers the length of ndim
giving the size in bytes to
access each element for each dimension of the array.
readonly
A bool indicating whether the memory is read only.
New in version 2.5.
Python's with
statement supports the concept of a runtime context
defined by a context manager. This is implemented using two separate methods
that allow user-defined classes to define a runtime context that is entered
before the statement body is executed and exited when the statement ends.
The context management protocol consists of a pair of methods that need to be provided for a context manager object to define a runtime context:
contextmanager.__enter__()
Enter the runtime context and return either this object or another object
related to the runtime context. The value returned by this method is bound to
the identifier in the as
clause of with
statements using
this context manager.
An example of a context manager that returns itself is a file object. File
objects return themselves from __enter__() to allow open()
to be used as
the context expression in a with
statement.
An example of a context manager that returns a related object is the one
returned by decimal.localcontext()
. These managers set the active
decimal context to a copy of the original decimal context and then return the
copy. This allows changes to be made to the current decimal context in the body
of the with
statement without affecting code outside the
with
statement.
contextmanager.__exit__(exc_type, exc_val, exc_tb)
Exit the runtime context and return a Boolean flag indicating if any exception
that occurred should be suppressed. If an exception occurred while executing the
body of the with
statement, the arguments contain the exception type,
value and traceback information. Otherwise, all three arguments are None
.
Returning a true value from this method will cause the with
statement
to suppress the exception and continue execution with the statement immediately
following the with
statement. Otherwise the exception continues
propagating after this method has finished executing. Exceptions that occur
during execution of this method will replace any exception that occurred in the
body of the with
statement.
The exception passed in should never be reraised explicitly - instead, this
method should return a false value to indicate that the method completed
successfully and does not want to suppress the raised exception. This allows
context management code (such as contextlib.nested
) to easily detect whether
or not an __exit__()
method has actually failed.
Python defines several context managers to support easy thread synchronisation,
prompt closure of files or other objects, and simpler manipulation of the active
decimal arithmetic context. The specific types are not treated specially beyond
their implementation of the context management protocol. See the
contextlib
module for some examples.
Python's generators and the contextlib.contextmanager
decorator
provide a convenient way to implement these protocols. If a generator function is
decorated with the contextlib.contextmanager
decorator, it will return a
context manager implementing the necessary __enter__()
and
__exit__()
methods, rather than the iterator produced by an undecorated
generator function.
Note that there is no specific slot for any of these methods in the type structure for Python objects in the Python/C API. Extension types wanting to define these methods must provide them as a normal Python accessible method. Compared to the overhead of setting up the runtime context, the overhead of a single class dictionary lookup is negligible.
The interpreter supports several other kinds of objects. Most of these support only one or two operations.
The only special operation on a module is attribute access: m.name
, where
m is a module and name accesses a name defined in m's symbol table.
Module attributes can be assigned to. (Note that the import
statement is not, strictly speaking, an operation on a module object; import
foo
does not require a module object named foo to exist, rather it requires
an (external) definition for a module named foo somewhere.)
A special attribute of every module is __dict__
. This is the
dictionary containing the module's symbol table. Modifying this dictionary will
actually change the module's symbol table, but direct assignment to the
__dict__
attribute is not possible (you can write
m.__dict__['a'] = 1
, which defines m.a
to be 1
, but you can't write
m.__dict__ = {}
). Modifying __dict__
directly is
not recommended.
Modules built into the interpreter are written like this: <module 'sys'
(built-in)>
. If loaded from a file, they are written as <module 'os' from
'/usr/local/lib/pythonX.Y/os.pyc'>
.
See Objects, values and types and Class definitions for these.
Function objects are created by function definitions. The only operation on a
function object is to call it: func(argument-list)
.
There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types.
See Function definitions for more information.
Methods are functions that are called using the attribute notation. There are
two flavors: built-in methods (such as append()
on lists) and class
instance methods. Built-in methods are described with the types that support
them.
The implementation adds two special read-only attributes to class instance
methods: m.im_self
is the object on which the method operates, and
m.im_func
is the function implementing the method. Calling m(arg-1,
arg-2, ..., arg-n)
is completely equivalent to calling m.im_func(m.im_self,
arg-1, arg-2, ..., arg-n)
.
Class instance methods are either bound or unbound, referring to whether the
method was accessed through an instance or a class, respectively. When a method
is unbound, its im_self
attribute will be None
and if called, an
explicit self
object must be passed as the first argument. In this case,
self
must be an instance of the unbound method's class (or a subclass of
that class), otherwise a TypeError
is raised.
Like function objects, methods objects support getting arbitrary attributes.
However, since method attributes are actually stored on the underlying function
object (meth.im_func
), setting method attributes on either bound or unbound
methods is disallowed. Attempting to set an attribute on a method results in
an AttributeError
being raised. In order to set a method attribute, you
need to explicitly set it on the underlying function object:
>>> class C:
... def method(self):
... pass
...
>>> c = C()
>>> c.method.whoami = 'my name is method' # can't set on the method
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'instancemethod' object has no attribute 'whoami'
>>> c.method.im_func.whoami = 'my name is method'
>>> c.method.whoami
'my name is method'
See The standard type hierarchy for more information.
Code objects are used by the implementation to represent "pseudo-compiled"
executable Python code such as a function body. They differ from function
objects because they don't contain a reference to their global execution
environment. Code objects are returned by the built-in compile()
function
and can be extracted from function objects through their func_code
attribute. See also the code
module.
A code object can be executed or evaluated by passing it (instead of a source
string) to the exec
statement or the built-in eval()
function.
See The standard type hierarchy for more information.
Type objects represent the various object types. An object's type is accessed
by the built-in function type()
. There are no special operations on
types. The standard module types
defines names for all standard built-in
types.
Types are written like this: <type 'int'>
.
This object is returned by functions that don't explicitly return a value. It
supports no special operations. There is exactly one null object, named
None
(a built-in name).
It is written as None
.
This object is used by extended slice notation (see Slicings). It
supports no special operations. There is exactly one ellipsis object, named
Ellipsis
(a built-in name).
It is written as Ellipsis
. When in a subscript, it can also be written as
...
, for example seq[...]
.
This object is returned from comparisons and binary operations when they are asked to operate on types they don't support. See Comparisons for more information.
It is written as NotImplemented
.
Boolean values are the two constant objects False
and True
. They are
used to represent truth values (although other values can also be considered
false or true). In numeric contexts (for example when used as the argument to
an arithmetic operator), they behave like the integers 0 and 1, respectively.
The built-in function bool()
can be used to convert any value to a
Boolean, if the value can be interpreted as a truth value (see section
Truth Value Testing above).
They are written as False
and True
, respectively.
See The standard type hierarchy for this information. It describes stack frame objects, traceback objects, and slice objects.
The implementation adds a few special read-only attributes to several object
types, where they are relevant. Some of these are not reported by the
dir()
built-in function.
object.__dict__
A dictionary or other mapping object used to store an object's (writable) attributes.
object.__methods__
Deprecated since version 2.2: Use the built-in function dir()
to get a list of an object's attributes.
This attribute is no longer available.
object.__members__
Deprecated since version 2.2: Use the built-in function dir()
to get a list of an object's attributes.
This attribute is no longer available.
instance.__class__
The class to which a class instance belongs.
class.__bases__
The tuple of base classes of a class object.
definition.__name__
The name of the class, type, function, method, descriptor, or generator instance.
The following attributes are only supported by new-style classes.
class.__mro__
This attribute is a tuple of classes that are considered when looking for base classes during method resolution.
class.mro()
This method can be overridden by a metaclass to customize the method
resolution order for its instances. It is called at class instantiation, and
its result is stored in __mro__
.
class.__subclasses__()
Each new-style class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. Example:
>>> int.__subclasses__()
[<type 'bool'>]