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w is as for now a proof of concept of an independant web debugger for wsgi project (implemented as a wsgi middleware).

Unlike other web debuggers, w is based on the python bdb debugger framework. (The one used by pdb)

This allows step by step debugging during the rendering of the page as well as exception inspection in the very state the exception occured.

How is it possible ?

WebSockets! All debug information between web page and web server transits throught a websocket opened on a random port. Server request processing is blocked by the websocket and then resumed when the debugging is over.


This is still far from working perfectly, it has a lot of known issues and can easily break your application but the page debugging does work.

Random disclaimer warning

This python thing will probably eat your cat.


    $ pip install w


To try it you can simply run the script which is a flask application which will be accessible at http://localhost:1984/, http://localhost:1984/wtf for step by step testing.

To try it on another wsgi application, use the W middleware:

    from w import W
    wsgi_app = Whathever_wsgi_server_lib()
    my_app = W(wsgi_app)

Using flask:

    from w import W
    app = Flask(__name__)
    app.wsgi_app = W(app.wsgi_app)

You can now put some breakpoint in a request code:

    do_something()  # Will break here

Once you are in a breakpoint or in an exception, you can eval all you want in the prompt under the code. Multi-lines are partially supported using [Shift] + [Enter].

As of now the following special commands are supported during breakpoint:

  • .s or [Ctrl] + [↓]: Step into
  • .n or [Ctrl] + [→]: Step over (Next)
  • .c or [Ctrl] + [↑]: Step out (Return)
  • .c or [Ctrl] + [←]: Continue
  • .q: Quit

NB: Hotkeys are purposedly not triggered in the eval prompt to avoid conflicts when typing.


Florian Mounier @ Kozea


This library is licensed under GPLv3

w - An improbable web debugger through WebSockets

w Copyright (C) 2012  Florian Mounier, Kozea

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <>.