Flask debugging tip #3

If you need to know which Flask routes are configured you can execute the following command.

print flask_app.url_map

More Flask debugging tricks

I was busy debugging a rest services I’m building in Flask and Flask-Restplus. I needed a way to log the request and response. I found some code that can be used as a wrapper of the WSGI application that does just this.

Create a file called dubug.py that contains the following code.

import pprint

class RequestLoggingWrapper(object):
    def __init__(self, app):
        self._app = app

    def __call__(self, environ, resp):
        errorlog = environ['wsgi.errors']
        pprint.pprint(('REQUEST', environ), stream=errorlog)

        def log_response(status, headers, *args):
            pprint.pprint(('RESPONSE', status, headers), stream=errorlog)
            return resp(status, headers, *args)

        return self._app(environ, log_response)

This wrapper works independently from Flask and wraps the WSGI application. It shows exactly what request is going in and what response is going out.

When running Flask with the built-in server you can use it as follows.

from debug import RequestLoggingWrapper

if __name__ == '__main__':
    app.wsgi_app = RequestLoggingWrapper(app.wsgi_app)

The output goes to the wgi.error stream. For the built Flask server it is printed to stderr.

Flask request debugging

I needed to debug the Flask request. After googeling around for a while I ran into a cool trick on Stack Overflow using the pprint module. The pprint module provides a capability to “pretty-print” arbitrary Python data structures in a form which can be used as input to the interpreter.

import pprint
str = pprint.pformat(request.environ, depth=5)

This same trick can be used with all the Flask variables.

  • request.args: the key/value pairs in the URL query string
  • request.form: the key/value pairs in the body, as sent by a HTML POST form
  • request.files: the files in the body, which Flask keeps separate from form
  • request.values: combined args and form, preferring args if keys overlap

Cheatsheet virtualenvwrapper

This is a cheatsheet for virtualwrapper.

Install virtualenvwrapper

pip install virtualenv

pip install virtualenvwrapper-win

Set environment variable

Add an environment variable WORKON_HOME to specify the path to store environments. By default, this is %USERPROFILE%\Envs. I have set it to the following.


Main commands

mkvirtualenv <name>


workon <name>



add2virtualenv <full or relative path>

setprojectdir <full or relative path>





Cleanup the default Python environment

I have been messing around with Python these last few weeks. Playing with Python means installing a lot of packages. I did this without using virtualenv, so my default Python environment had a lot of packages installed globally.

I now have a couple of projects that I want to work on and I what each project to have its own clean environment. That means using virtualenv. When running virtualenv without with the option --no-site-packages all the packages that are installed globally are included in the virtualenv.

I wanted to remove all the global packages to have a clean default environment. I did this by running the following command.


pip freeze > remove.txt && pip uninstall -y -r remove.txt && del remove.txt


pip freeze > remove.txt && pip uninstall -y -r remove.txt && rm remove.txt

After cleaning up the default envrionment by removing all the global packages don’t forget to install virtualenv en virtualenvwrapper-win by running the following command.

pip install virtualenvwrapper-win

For quick instructions of the main commands please check out virtualenvwrapper-win Github page.