In Python, all exceptions must be instances of a class that derives from
BaseException
. In a try
statement with an except
clause that mentions a particular class, that clause also handles any exception
classes derived from that class (but not exception classes from which it is
derived). Two exception classes that are not related via subclassing are never
equivalent, even if they have the same name.
The built-in exceptions listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an "associated value" indicating the detailed cause of the error. This may be a string or a tuple of several items of information (e.g., an error code and a string explaining the code). The associated value is usually passed as arguments to the exception class's constructor.
User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition "just like" the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error.
The built-in exception classes can be subclassed to define new exceptions;
programmers are encouraged to derive new exceptions from the Exception
class or one of its subclasses, and not from BaseException
. More
information on defining exceptions is available in the Python Tutorial under
User-defined Exceptions.
When raising (or re-raising) an exception in an except
or
finally
clause
__context__
is automatically set to the last exception caught; if the
new exception is not handled the traceback that is eventually displayed will
include the originating exception(s) and the final exception.
When raising a new exception (rather than using a bare raise
to re-raise
the exception currently being handled), the implicit exception context can be
supplemented with an explicit cause by using from
with
raise
:
raise new_exc from original_exc
The expression following from
must be an exception or None
. It
will be set as __cause__
on the raised exception. Setting
__cause__
also implicitly sets the __suppress_context__
attribute to True
, so that using raise new_exc from None
effectively replaces the old exception with the new one for display
purposes (e.g. converting KeyError
to AttributeError
, while
leaving the old exception available in __context__
for introspection
when debugging.
The default traceback display code shows these chained exceptions in
addition to the traceback for the exception itself. An explicitly chained
exception in __cause__
is always shown when present. An implicitly
chained exception in __context__
is shown only if __cause__
is None
and __suppress_context__
is false.
In either case, the exception itself is always shown after any chained exceptions so that the final line of the traceback always shows the last exception that was raised.
The following exceptions are used mostly as base classes for other exceptions.
exception BaseException
The base class for all built-in exceptions. It is not meant to be directly
inherited by user-defined classes (for that, use Exception
). If
str()
is called on an instance of this class, the representation of
the argument(s) to the instance are returned, or the empty string when
there were no arguments.
args
The tuple of arguments given to the exception constructor. Some built-in
exceptions (like OSError
) expect a certain number of arguments and
assign a special meaning to the elements of this tuple, while others are
usually called only with a single string giving an error message.
with_traceback(tb)
This method sets tb as the new traceback for the exception and returns the exception object. It is usually used in exception handling code like this:
try:
...
except SomeException:
tb = sys.exc_info()[2]
raise OtherException(...).with_traceback(tb)
exception Exception
All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.
exception ArithmeticError
The base class for those built-in exceptions that are raised for various
arithmetic errors: OverflowError
, ZeroDivisionError
,
FloatingPointError
.
exception BufferError
Raised when a buffer related operation cannot be performed.
exception LookupError
The base class for the exceptions that are raised when a key or index used on
a mapping or sequence is invalid: IndexError
, KeyError
. This
can be raised directly by codecs.lookup()
.
The following exceptions are the exceptions that are usually raised.
exception AssertionError
Raised when an assert
statement fails.
exception AttributeError
Raised when an attribute reference (see Attribute references) or
assignment fails. (When an object does not support attribute references or
attribute assignments at all, TypeError
is raised.)
exception EOFError
Raised when the input()
function hits an end-of-file condition (EOF)
without reading any data. (N.B.: the io.IOBase.read()
and
io.IOBase.readline()
methods return an empty string when they hit EOF.)
exception FloatingPointError
Raised when a floating point operation fails. This exception is always defined,
but can only be raised when Python is configured with the
--with-fpectl
option, or the WANT_SIGFPE_HANDLER
symbol is
defined in the pyconfig.h
file.
exception GeneratorExit
Raised when a generator or coroutine is closed;
see generator.close()
and coroutine.close()
. It
directly inherits from BaseException
instead of Exception
since
it is technically not an error.
exception ImportError
Raised when the import
statement has troubles trying to
load a module. Also raised when the "from list" in from ... import
has a name that cannot be found.
The name
and path
attributes can be set using keyword-only
arguments to the constructor. When set they represent the name of the module
that was attempted to be imported and the path to any file which triggered
the exception, respectively.
Changed in version 3.3: Added the name
and path
attributes.
exception ModuleNotFoundError
A subclass of ImportError
which is raised by import
when a module could not be located. It is also raised when None
is found in sys.modules
.
New in version 3.6.
exception IndexError
Raised when a sequence subscript is out of range. (Slice indices are
silently truncated to fall in the allowed range; if an index is not an
integer, TypeError
is raised.)
exception KeyError
Raised when a mapping (dictionary) key is not found in the set of existing keys.
exception KeyboardInterrupt
Raised when the user hits the interrupt key (normally Control-C
or
Delete
). During execution, a check for interrupts is made
regularly. The exception inherits from BaseException
so as to not be
accidentally caught by code that catches Exception
and thus prevent
the interpreter from exiting.
exception MemoryError
Raised when an operation runs out of memory but the situation may still be
rescued (by deleting some objects). The associated value is a string indicating
what kind of (internal) operation ran out of memory. Note that because of the
underlying memory management architecture (C's malloc()
function), the
interpreter may not always be able to completely recover from this situation; it
nevertheless raises an exception so that a stack traceback can be printed, in
case a run-away program was the cause.
exception NameError
Raised when a local or global name is not found. This applies only to unqualified names. The associated value is an error message that includes the name that could not be found.
exception NotImplementedError
This exception is derived from RuntimeError
. In user defined base
classes, abstract methods should raise this exception when they require
derived classes to override the method, or while the class is being
developed to indicate that the real implementation still needs to be added.
Note
It should not be used to indicate that an operator or method is not
meant to be supported at all -- in that case either leave the operator /
method undefined or, if a subclass, set it to None
.
Note
NotImplementedError
and NotImplemented
are not interchangeable,
even though they have similar names and purposes. See
NotImplemented
for details on when to use it.
exception OSError([arg])
exception OSError(errno, strerror[, filename[, winerror[, filename2]]])
This exception is raised when a system function returns a system-related error, including I/O failures such as "file not found" or "disk full" (not for illegal argument types or other incidental errors).
The second form of the constructor sets the corresponding attributes,
described below. The attributes default to None
if not
specified. For backwards compatibility, if three arguments are passed,
the args
attribute contains only a 2-tuple
of the first two constructor arguments.
The constructor often actually returns a subclass of OSError
, as
described in OS exceptions below. The particular subclass depends on
the final errno
value. This behaviour only occurs when
constructing OSError
directly or via an alias, and is not
inherited when subclassing.
errno
A numeric error code from the C variable errno
.
winerror
Under Windows, this gives you the native
Windows error code. The errno
attribute is then an approximate
translation, in POSIX terms, of that native error code.
Under Windows, if the winerror constructor argument is an integer,
the errno
attribute is determined from the Windows error code,
and the errno argument is ignored. On other platforms, the
winerror argument is ignored, and the winerror
attribute
does not exist.
strerror
The corresponding error message, as provided by
the operating system. It is formatted by the C
functions perror()
under POSIX, and FormatMessage()
under Windows.
filename
filename2
For exceptions that involve a file system path (such as open()
or
os.unlink()
), filename
is the file name passed to the function.
For functions that involve two file system paths (such as
os.rename()
), filename2
corresponds to the second
file name passed to the function.
Changed in version 3.3: EnvironmentError
, IOError
, WindowsError
,
socket.error
, select.error
and
mmap.error
have been merged into OSError
, and the
constructor may return a subclass.
Changed in version 3.4: The filename
attribute is now the original file name passed to
the function, instead of the name encoded to or decoded from the
filesystem encoding. Also, the filename2 constructor argument and
attribute was added.
exception OverflowError
Raised when the result of an arithmetic operation is too large to be
represented. This cannot occur for integers (which would rather raise
MemoryError
than give up). However, for historical reasons,
OverflowError is sometimes raised for integers that are outside a required
range. Because of the lack of standardization of floating point exception
handling in C, most floating point operations are not checked.
exception RecursionError
This exception is derived from RuntimeError
. It is raised when the
interpreter detects that the maximum recursion depth (see
sys.getrecursionlimit()
) is exceeded.
New in version 3.5: Previously, a plain RuntimeError
was raised.
exception ReferenceError
This exception is raised when a weak reference proxy, created by the
weakref.proxy()
function, is used to access an attribute of the referent
after it has been garbage collected. For more information on weak references,
see the weakref
module.
exception RuntimeError
Raised when an error is detected that doesn't fall in any of the other categories. The associated value is a string indicating what precisely went wrong.
exception StopIteration
Raised by built-in function next()
and an iterator's
__next__()
method to signal that there are no further
items produced by the iterator.
The exception object has a single attribute value
, which is
given as an argument when constructing the exception, and defaults
to None
.
When a generator or coroutine function
returns, a new StopIteration
instance is
raised, and the value returned by the function is used as the
value
parameter to the constructor of the exception.
If a generator function defined in the presence of a from __future__
import generator_stop
directive raises StopIteration
, it will be
converted into a RuntimeError
(retaining the StopIteration
as the new exception's cause).
Changed in version 3.3: Added value
attribute and the ability for generator functions to
use it to return a value.
Changed in version 3.5: Introduced the RuntimeError transformation.
exception StopAsyncIteration
Must be raised by __anext__()
method of an
asynchronous iterator object to stop the iteration.
New in version 3.5.
exception SyntaxError
Raised when the parser encounters a syntax error. This may occur in an
import
statement, in a call to the built-in functions exec()
or eval()
, or when reading the initial script or standard input
(also interactively).
Instances of this class have attributes filename
, lineno
,
offset
and text
for easier access to the details. str()
of the exception instance returns only the message.
exception IndentationError
Base class for syntax errors related to incorrect indentation. This is a
subclass of SyntaxError
.
exception TabError
Raised when indentation contains an inconsistent use of tabs and spaces.
This is a subclass of IndentationError
.
exception SystemError
Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms).
You should report this to the author or maintainer of your Python interpreter.
Be sure to report the version of the Python interpreter (sys.version
; it is
also printed at the start of an interactive Python session), the exact error
message (the exception's associated value) and if possible the source of the
program that triggered the error.
exception SystemExit
This exception is raised by the sys.exit()
function. It inherits from
BaseException
instead of Exception
so that it is not accidentally
caught by code that catches Exception
. This allows the exception to
properly propagate up and cause the interpreter to exit. When it is not
handled, the Python interpreter exits; no stack traceback is printed. The
constructor accepts the same optional argument passed to sys.exit()
.
If the value is an integer, it specifies the system exit status (passed to
C's exit()
function); if it is None
, the exit status is zero; if
it has another type (such as a string), the object's value is printed and
the exit status is one.
A call to sys.exit()
is translated into an exception so that clean-up
handlers (finally
clauses of try
statements) can be
executed, and so that a debugger can execute a script without running the risk
of losing control. The os._exit()
function can be used if it is
absolutely positively necessary to exit immediately (for example, in the child
process after a call to os.fork()
).
code
The exit status or error message that is passed to the constructor.
(Defaults to None
.)
exception TypeError
Raised when an operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch.
This exception may be raised by user code to indicate that an attempted
operation on an object is not supported, and is not meant to be. If an object
is meant to support a given operation but has not yet provided an
implementation, NotImplementedError
is the proper exception to raise.
Passing arguments of the wrong type (e.g. passing a list
when an
int
is expected) should result in a TypeError
, but passing
arguments with the wrong value (e.g. a number outside expected boundaries)
should result in a ValueError
.
exception UnboundLocalError
Raised when a reference is made to a local variable in a function or method, but
no value has been bound to that variable. This is a subclass of
NameError
.
exception UnicodeError
Raised when a Unicode-related encoding or decoding error occurs. It is a
subclass of ValueError
.
UnicodeError
has attributes that describe the encoding or decoding
error. For example, err.object[err.start:err.end]
gives the particular
invalid input that the codec failed on.
encoding
The name of the encoding that raised the error.
reason
A string describing the specific codec error.
object
The object the codec was attempting to encode or decode.
start
The first index of invalid data in object
.
end
The index after the last invalid data in object
.
exception UnicodeEncodeError
Raised when a Unicode-related error occurs during encoding. It is a subclass of
UnicodeError
.
exception UnicodeDecodeError
Raised when a Unicode-related error occurs during decoding. It is a subclass of
UnicodeError
.
exception UnicodeTranslateError
Raised when a Unicode-related error occurs during translating. It is a subclass
of UnicodeError
.
exception ValueError
Raised when a built-in operation or function receives an argument that has the
right type but an inappropriate value, and the situation is not described by a
more precise exception such as IndexError
.
exception ZeroDivisionError
Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation.
The following exceptions are kept for compatibility with previous versions;
starting from Python 3.3, they are aliases of OSError
.
exception EnvironmentError
exception IOError
exception WindowsError
Only available on Windows.
The following exceptions are subclasses of OSError
, they get raised
depending on the system error code.
exception BlockingIOError
Raised when an operation would block on an object (e.g. socket) set
for non-blocking operation.
Corresponds to errno
EAGAIN
, EALREADY
,
EWOULDBLOCK
and EINPROGRESS
.
In addition to those of OSError
, BlockingIOError
can have
one more attribute:
characters_written
An integer containing the number of characters written to the stream
before it blocked. This attribute is available when using the
buffered I/O classes from the io
module.
exception ChildProcessError
Raised when an operation on a child process failed.
Corresponds to errno
ECHILD
.
exception ConnectionError
A base class for connection-related issues.
Subclasses are BrokenPipeError
, ConnectionAbortedError
,
ConnectionRefusedError
and ConnectionResetError
.
exception BrokenPipeError
A subclass of ConnectionError
, raised when trying to write on a
pipe while the other end has been closed, or trying to write on a socket
which has been shutdown for writing.
Corresponds to errno
EPIPE
and ESHUTDOWN
.
exception ConnectionAbortedError
A subclass of ConnectionError
, raised when a connection attempt
is aborted by the peer.
Corresponds to errno
ECONNABORTED
.
exception ConnectionRefusedError
A subclass of ConnectionError
, raised when a connection attempt
is refused by the peer.
Corresponds to errno
ECONNREFUSED
.
exception ConnectionResetError
A subclass of ConnectionError
, raised when a connection is
reset by the peer.
Corresponds to errno
ECONNRESET
.
exception FileExistsError
Raised when trying to create a file or directory which already exists.
Corresponds to errno
EEXIST
.
exception FileNotFoundError
Raised when a file or directory is requested but doesn't exist.
Corresponds to errno
ENOENT
.
exception InterruptedError
Raised when a system call is interrupted by an incoming signal.
Corresponds to errno
EINTR
.
Changed in version 3.5: Python now retries system calls when a syscall is interrupted by a
signal, except if the signal handler raises an exception (see PEP 475
for the rationale), instead of raising InterruptedError
.
exception IsADirectoryError
Raised when a file operation (such as os.remove()
) is requested
on a directory.
Corresponds to errno
EISDIR
.
exception NotADirectoryError
Raised when a directory operation (such as os.listdir()
) is requested
on something which is not a directory.
Corresponds to errno
ENOTDIR
.
exception PermissionError
Raised when trying to run an operation without the adequate access
rights - for example filesystem permissions.
Corresponds to errno
EACCES
and EPERM
.
exception ProcessLookupError
Raised when a given process doesn't exist.
Corresponds to errno
ESRCH
.
exception TimeoutError
Raised when a system function timed out at the system level.
Corresponds to errno
ETIMEDOUT
.
New in version 3.3: All the above OSError
subclasses were added.
See also
PEP 3151 - Reworking the OS and IO exception hierarchy
The following exceptions are used as warning categories; see the warnings
module for more information.
exception Warning
Base class for warning categories.
exception UserWarning
Base class for warnings generated by user code.
exception DeprecationWarning
Base class for warnings about deprecated features.
exception PendingDeprecationWarning
Base class for warnings about features which will be deprecated in the future.
exception SyntaxWarning
Base class for warnings about dubious syntax.
exception RuntimeWarning
Base class for warnings about dubious runtime behavior.
exception FutureWarning
Base class for warnings about constructs that will change semantically in the future.
exception ImportWarning
Base class for warnings about probable mistakes in module imports.
exception UnicodeWarning
Base class for warnings related to Unicode.
exception BytesWarning
exception ResourceWarning
Base class for warnings related to resource usage.
New in version 3.2.
The class hierarchy for built-in exceptions is:
BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StopAsyncIteration
+-- ArithmeticError
| +-- FloatingPointError
| +-- OverflowError
| +-- ZeroDivisionError
+-- AssertionError
+-- AttributeError
+-- BufferError
+-- EOFError
+-- ImportError
+-- ModuleNotFoundError
+-- LookupError
| +-- IndexError
| +-- KeyError
+-- MemoryError
+-- NameError
| +-- UnboundLocalError
+-- OSError
| +-- BlockingIOError
| +-- ChildProcessError
| +-- ConnectionError
| | +-- BrokenPipeError
| | +-- ConnectionAbortedError
| | +-- ConnectionRefusedError
| | +-- ConnectionResetError
| +-- FileExistsError
| +-- FileNotFoundError
| +-- InterruptedError
| +-- IsADirectoryError
| +-- NotADirectoryError
| +-- PermissionError
| +-- ProcessLookupError
| +-- TimeoutError
+-- ReferenceError
+-- RuntimeError
| +-- NotImplementedError
| +-- RecursionError
+-- SyntaxError
| +-- IndentationError
| +-- TabError
+-- SystemError
+-- TypeError
+-- ValueError
| +-- UnicodeError
| +-- UnicodeDecodeError
| +-- UnicodeEncodeError
| +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+-- ResourceWarning