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.

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.DebuggedApplication(app, evalex=False, request_key='werkzeug.request', console_path='/console', console_init_func=None, show_hidden_frames=False)

Enables debugging support for a given application:

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

The evalex keyword argument allows evaluating expressions in a traceback’s frame context.

  • 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 True.

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 evalex 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 dump() 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.

Pasting Errors

If you click on the Traceback title the traceback switches over to a text based one. The text based one can be pasted to paste.pocoo.org with one click.