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About backlash

backlash is a swiss army knife for web applications debugging, which provides:

  • An Interactive In Browser Debugger based on a Werkzeug Debugger fork ported to WebOb
  • Crash reporting by email and on Sentry
  • Slow requests reporting by email and on Sentry.

Backlash was born as a replacement for WebError in TurboGears2.3 versions.


backlash can be installed from pypi:

pip install backlash

should just work for most of the users

Debugging and Console

Backlash supports both debugging applications on crash and realtime console, both are based on the Werkzeug Debugger and adapted to work with WebOb.

The debugging function is provided by the DebuggedApplication middleware, wrapping your application with this middleware will intercept any exception and display the traceback and an interactive console in your browser.

An interactive console will also be always available at /__console__ path.

Context Injectors

The DebuggedApplication middleware also makes possible to provide one or more context injectors, those are simple python functions that will be called when an exception is raised to retrieve the context to store and make back available during debugging.

Context injectors have to return a dictionary which will be merged into the current request context, the request context itself will be made available inside the debugger as the ctx object.

This feature is used for example by TurboGears to provide back some of the objects which were available during execution like the current request.


The DebuggedApplication middleware is used by TurboGears in the following way:

def _turbogears_backlash_context(environ):
    tgl = environ.get('tg.locals')
    return {'request':getattr(tgl, 'request', None)}

app = backlash.DebuggedApplication(app, context_injectors=[_turbogears_backlash_context])

Exception Tracing

The TraceErrorsMiddleware provides a WSGI middleware that intercepts any exception raised during execution, retrieves a traceback object and provides it to one or more reporters to log the error.

By default the EmailReporter and SentryReporter are provided to send error reports by email and on Sentry.

The EmailReporter supports most of the options WebError ErrorMiddleware to provide some kind of backward compatibility and make possible a quick transition.

While this function is easily replicable using the python logging SMTPHandler, the TraceErrorsMiddleware is explicitly meant for web applications crash reporting which has the benefit of being able to provide more complete information and keep a clear and separate process in managing errors.


The TraceErrorsMiddleware is used by TurboGears in the following way:

from backlash.trace_errors import EmailReporter

def _turbogears_backlash_context(environ):
   tgl = environ.get('tg.locals')
   return {'request':getattr(tgl, 'request', None)}

app = backlash.TraceErrorsMiddleware(app, [EmailReporter(**errorware)],

Slow Requests Tracing

The TraceSlowRequestsMiddleware provides a WSGI middleware that tracks requests execution time and reports requests that took more than a specified interval to complete (by default 25 seconds).

It is also possible to exclude a list of paths that start with a specified string to avoid reporting long polling connections or other kind of requests that are expected to have a long life spawn.


The TraceSlowRequestsMiddleware is used by TurboGears in the following way:

from backlash.trace_errors import EmailReporter

def _turbogears_backlash_context(environ):
   tgl = environ.get('tg.locals')
   return {'request':getattr(tgl, 'request', None)}

app = backlash.TraceSlowRequestsMiddleware(app, [EmailReporter(**errorware)],
                                           interval=25, exclude_paths=None,


Standalone WebOb port of the Werkzeug Debugger with Python3 support born as a WebError replacement for TurboGears2




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