Toggle navigation

Debugging Applications

Depending on the WSGI gateway/server, exceptions are handled differently. But most of the time, exceptions go to stderr or the error log.

Since this is not the best debugging environment, Werkzeug provides a WSGI middleware that renders nice debugging tracebacks, optionally with an AJAX based debugger (which allows to execute code in the context of the traceback’s frames).

The interactive debugger however does not work in forking environments which makes it nearly impossible to use on production servers. Also the debugger allows the execution of arbitrary code which makes it a major security risk and must never be used on production machines because of that. We cannot stress this enough. Do not enable this in production.

Enabling the Debugger

You can enable the debugger by wrapping the application in a DebuggedApplication middleware. Additionally there are parameters to the run_simple() function to enable it because this is a common task during development.

class werkzeug.debug.DebuggedApplication(app, evalex=False, request_key='werkzeug.request', console_path='/console', console_init_func=None, show_hidden_frames=False, lodgeit_url=None, pin_security=True, pin_logging=True)

Enables debugging support for a given application:

from werkzeug.debug import DebuggedApplication
from myapp import app
app = DebuggedApplication(app, evalex=True)

The [UNKNOWN NODE title_reference] keyword argument allows evaluating expressions in a traceback’s frame context.

New in version 0.9: The [UNKNOWN NODE title_reference] parameter was deprecated.

  • app – the WSGI application to run debugged.
  • evalex – enable exception evaluation feature (interactive debugging). This requires a non-forking server.
  • request_key – The key that points to the request object in ths environment. This parameter is ignored in current versions.
  • console_path – the URL for a general purpose console.
  • console_init_func – the function that is executed before starting the general purpose console. The return value is used as initial namespace.
  • show_hidden_frames – by default hidden traceback frames are skipped. You can show them by setting this parameter to [UNKNOWN NODE title_reference].
  • pin_security – can be used to disable the pin based security system.
  • pin_logging – enables the logging of the pin system.

Using the Debugger

Once enabled and an error happens during a request you will see a detailed traceback instead of a general “internal server error”. If you have the [UNKNOWN NODE title_reference] feature enabled you can also get a traceback for every frame in the traceback by clicking on the console icon.

Once clicked a console opens where you can execute Python code in:

a screenshot of the interactive debugger

Inside the interactive consoles you can execute any kind of Python code. Unlike regular Python consoles the output of the object reprs is colored and stripped to a reasonable size by default. If the output is longer than what the console decides to display a small plus sign is added to the repr and a click will expand the repr.

To display all variables that are defined in the current frame you can use the [UNKNOWN NODE title_reference] function. You can call it without arguments to get a detailed list of all variables and their values, or with an object as argument to get a detailed list of all the attributes it has.

Debugger PIN

Starting with Werkzeug 0.11 the debugger is additionally protected by a PIN. This is a security helper to make it less likely for the debugger to be exploited in production as it has happened to people to keep the debugger active. The PIN based authentication is enabled by default.

When the debugger comes up, on first usage it will prompt for a PIN that is printed to the command line. The PIN is generated in a stable way that is specific to the project. In some situations it might be not possible to generate a stable PIN between restarts in which case an explicit PIN can be provided through the environment variable WERKZEUG_DEBUG_PIN. This can be set to a number and will become the PIN. This variable can also be set to the value off to disable the PIN check entirely.

If the PIN is entered too many times incorrectly the server needs to be restarted.

This feature is not supposed to entirely secure the debugger. It’s intended to make it harder for an attacker to exploit the debugger. Never enable the debugger in production.

Pasting Errors

If you click on the [UNKNOWN NODE title_reference] title, the traceback switches over to a text based one. The text based one can be pasted to with one click.