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An improbable web debugger through WebSockets
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wdb - Web Debugger

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Colors are bad due to gif compression.


wdb is a full featured web debugger based on a client-server architecture.

The wdb server which is responsible of managing debugging instances along with browser connections (through websockets) is based on Tornado. The wdb clients allow step by step debugging, in-program python code execution, code edition (based on CodeMirror) setting breakpoints...

Due to this architecture, all of this is fully compatible with multithread and multiprocess programs.

wdb works with python 2 (2.6, 2.7), python 3 (3.2, 3.3, 3.4) and pypy. Even better, it is possible to debug a python 2 program with a wdb server running on python 3 and vice-versa or debug a program running on a computer with a debugging server running on another computer inside a web page on a third computer!

Even betterer, it is now possible to pause a currently running python process/thread using code injection from the web interface. (This requires gdb and ptrace enabled)

In other words it's a very enhanced version of pdb directly in your browser with nice features.


Global installation:

    $ pip install wdb.server

In virtualenv or with a different python installation:

    $ pip install wdb

(You must have the server installed and running)

Quick test

To try wdb, first you have to start the wdb server:

    $ &

Next run:

    $ python -m wdb

Wdb will open a debugging window right in your browser, paused at the beginning of your program.

You can access to http://localhost:1984/ to have an overview of the server.

NB: You have to start the server only once. Multiple Debugging sessions can be run simultaneously without problem.

This is not the only way to debug a program, see below.


Setting trace

To debug any program, with the server on, just add:

    import wdb

anywhere in your code. Your program will stop at the set_trace line. (Just like pdb)

Tracing code

To inspect your code on exception, you can do the following:

    from wdb import trace
    with trace():

Any exception during wrong_code will launch a debugging session.

You can also use the start_trace() and stop_trace methods. (It's better to put the stop_trace in a finally block to avoid tracing all your program after an exception.)

Debugging web servers

wdb provides some tools to make it work nicely with different webservers:

Wsgi servers

For wsgi servers you can use the WdbMiddleware:

    from wdb.ext import WdbMiddleware
    wsgi_app = Whathever_wsgi_server_lib()
    my_app = WdbMiddleware(wsgi_app)

or using Flask:

    from flask import Flask
    from wdb.ext import WdbMiddleware
    app = Flask(__name__)
    app.debug = True
    app.wsgi_app = WdbMiddleware(app.wsgi_app) # Disable builtin Werkzeug debugger

you can also use the Flask-Wdb extension

    from flask import Flask
    from flask_wdb import Wdb

    app = Flask(__name__)
    app.debug = True

or using django:

Add the middleware in your


    from django.core.wsgi import get_wsgi_application
    application = get_wsgi_application()

Add this:

    from wdb.ext import WdbMiddleware
    application = WdbMiddleware(application)

And in your, activate exception propagation:

    DEBUG = True

or using CherryPy:

import cherrypy
from wdb.ext import WdbMiddleware

class HelloWorld(object):
    def index(self):
        undefined_method() # This will fail
        return "Hello World!"

cherrypy.config.update({'global':{'request.throw_errors': True}})
app = cherrypy.Application(HelloWorld())
app.wsgiapp.pipeline.append(('debugger', WdbMiddleware))



In tornado, which is not a wsgi server, you can use the wdb_tornado function which will monkey patch the execute method on RequestHandlers:

    from wdb.ext import wdb_tornado
    from tornado.web import Application
    my_app = Application([(r"/", MainHandler)])
    if options.debug:

Page loading time become slow

If wdb slows down too much of your application (tracing all the things takes time), you can start it disabled with:

    my_app = WdbMiddleware(wsgi_app, start_disabled=True)  # or
    wdb_tornado(my_app, start_disabled=True)

Then when you get an exception just click on the on/off button.

Remote debugging

You can easily do remote debugging with wdb:

Let's say you want to run a program on computer A and you want to debug it on computer B.

Start wdb server on computer A and launch this:

    WDB_NO_BROWSER_AUTO_OPEN=True python -m wdb

And open a browser on computer B at the url given by wdb log.

Now you can also run wdb server on a computer C and run on computer A:

    WDB_NO_BROWSER_AUTO_OPEN=True WDB_SOCKET_SERVER=computerC.addr WDB_SOCKET_PORT=19840 python -m wdb

And go with computer B to http://computerC/debug/session/[uuid in log] there you can step into running in computer A. Yay !

You can use different configurations:

See --help for changing ports on server and these environnement vars for wdb instances:

WDB_SOCKET_SERVER         # WDB server host
WDB_SOCKET_PORT           # WDB server socket port
WDB_WEB_SERVER            # WDB server host for browser openning
WDB_WEB_PORT              # WDB server http port
WDB_NO_BROWSER_AUTO_OPEN  # To disable the automagic browser openning (which can't be done if the browser is not on the same machine)

In browser usage

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] + [↓] or [F11]    : Step into
* .n or [Ctrl] + [→] or [F10]    : Step over (Next)
* .r or [Ctrl] + [↑] or [F9]     : Step out (Return)
* .c or [Ctrl] + [←] or [F8]     : Continue
* .u or [F7]                     : Until (Next over loops)
* .j lineno                      : Jump to lineno (Must be at bottom frame and in the same function)
* .b arg                         : Set a session breakpoint, see below for what arg can be*
* .t arg                         : Set a temporary breakpoint, arg follow the same syntax as .b
* .z arg                         : Delete existing breakpoint
* .l                             : List active breakpoints
* .f                             : Echo all typed commands in the current debugging session
* .d expression                  : Dump the result of expression in a table
* .w expression                  : Watch expression in curent file (Click on the name to remove)
* .q                             : Quit
* .h                             : Get some help
* .e                             : Toggle file edition mode
* .g                             : Clear prompt
* .i [mime/type;]expression      : Display the result in an embed, mime type is auto detected on linux and defaults to "text/html" otherwise
* iterable!sthg                  : If cutter is installed, executes cut(iterable).sthg
* expr >! file                   : Write the result of expr in file
* !< file                        : Eval the content of file
* [Enter]                        : Eval the current selected text in page, useful to eval code in the source
* * arg is using the following syntax:
*   [file/module][:lineno][#function][,condition]
* which means:
*   - [file]                    : Break if any line of `file` is executed
*   - [file]:lineno             : Break on `file` at `lineno`
*   - [file][:lineno],condition : Break on `file` at `lineno` if `condition` is True (ie: i == 10)
*   - [file]#function           : Break when inside `function` function
* File is always current file by default and you can also specify a module like `logging.config`.

You can also eval a variable in the source by middle clicking on it. You can add/remove a breakpoint by clicking on the line number.

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

Wdb Server

To see which debugging session are currently open, open your browser at http://localhost:1984/. You can also close crashed session.

From there you should also see all python process and their threads running and you can try to pause them during their execution to do step by step debugging and current variable inspection. This is highly experimental and requires gdb and a kernel with ptrace enabled to inject python code into a running python process. If you get ptrace: Operation not permitted. you will have to enable it.

Depending on your system it might work with:

echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

Make sure that wdb is installed for the python version running the program too.

Importing wdb each time is exhausting

Yes to avoid that, you can add a w builtin at the beginning of your application:

    from wdb.ext import add_w_builtin

you can now use the w object any where in your code:

    my_code()  # Stop next line
    with w.trace():


The Theme can be chosen in the server home page, or can be set at start like that:

    wdb.server --theme=light  # and if you disable the debugger
    WdbMiddleware(app, theme='light') # or
    wdb_tornado(app, theme='light')

If you like writing css feel free to make your own and pull request it to me, css is generated using libsass and grunt and most of the work will be setting variables.

Code completion

Wdb has dynamic code completion in eval prompt thanks to jedi.

Browser Support

wdb is based on websockets and since 3812417 its layout relies on flexbox, browser requirements are now :

  • Chrome >= 29
  • Firefox >= 22
  • IE >= 11

Better update you browser in any case. If it doesn't work with one of these versions please report it.


In Firefox opened debugging pages are not closed when done

It's a firefox config flag, visit about:config and set: dom.allow_scripts_to_close_windows to true


All contributions are more than welcomed, use the fork luke !

Or you still can use your money =)

Flattr gittip


Wdb is now partially tested, so if you want to contribute be sure to run the test suite:

    $ pip install pytest
    $ pip install -e client -e server
    $ cd test
    $ py.test

Feel free to add tests !


Florian Mounier @ Kozea


This library is licensed under GPLv3

wdb - An improbable web debugger through WebSockets

wdb Copyright (C) 2015  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 <>.
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