Working with the Shell

New in version 0.3.

One of the reasons everybody loves Python is the interactive shell. It basically allows you to execute Python commands in real time and immediately get results back. Flask itself does not come with an interactive shell, because it does not require any specific setup upfront, just import your application and start playing around.

There are however some handy helpers to make playing around in the shell a more pleasant experience. The main issue with interactive console sessions is that you’re not triggering a request like a browser does which means that g, request and others are not available. But the code you want to test might depend on them, so what can you do?

This is where some helper functions come in handy. Keep in mind however that these functions are not only there for interactive shell usage, but also for unittesting and other situations that require a faked request context.

Diving into Context Locals

Say you have a utility function that returns the URL the user should be redirected to. Imagine it would always redirect to the URL’s next parameter or the HTTP referrer or the index page:

from flask import request, url_for

def redirect_url():
    return request.args.get('next') or \
           request.referrer or \

As you can see, it accesses the request object. If you try to run this from a plain Python shell, this is the exception you will see:

>>> redirect_url()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'request'

That makes a lot of sense because we currently do not have a request we could access. So we have to make a request and bind it to the current context. The test_request_context method can create us a request context:

>>> ctx = app.test_request_context('/?next=')

This context can be used in two ways. Either with the with statement (which unfortunately is not very handy for shell sessions). The alternative way is to call the push and pop methods:

>>> ctx.push()

From that point onwards you can work with the request object:

>>> redirect_url()

Until you call pop:

>>> ctx.pop()
>>> redirect_url()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'request'

Firing Before/After Request

By just creating a request context, you still don’t have run the code that is normally run before a request. This probably results in your database being unavailable, the current user not being stored on the g object etc.

This however can easily be done yourself. Just call preprocess_request():

>>> ctx = app.test_request_context()
>>> ctx.push()
>>> app.preprocess_request()

Keep in mind that the preprocess_request() function might return a response object, in that case just ignore it.

To shutdown a request, you need to trick a bit before the after request functions (triggered by process_response()) operate on a response object:

>>> app.process_response(app.response_class())
<Response 0 bytes [200 OK]>
>>> ctx.pop()

Further Improving the Shell Experience

If you like the idea of experimenting in a shell, create yourself a module with stuff you want to star import into your interactive session. There you could also define some more helper methods for common things such as initializing the database, dropping tables etc.

Just put them into a module (like shelltools and import from there):

>>> from shelltools import *

Table Of Contents

Related Topics

This Page