How can one mock/stub python module like urllib

python unit-testing testing mocking
Dinoboff picture Dinoboff · Nov 17, 2008 · Viewed 50.7k times · Source

I need to test a function that needs to query a page on an external server using urllib.urlopen (it also uses urllib.urlencode). The server could be down, the page could change; I can't rely on it for a test.

What is the best way to control what urllib.urlopen returns?

Answer

Clint Miller picture Clint Miller · Nov 17, 2008

Another simple approach is to have your test override urllib's urlopen() function. For example, if your module has

import urllib

def some_function_that_uses_urllib():
    ...
    urllib.urlopen()
    ...

You could define your test like this:

import mymodule

def dummy_urlopen(url):
    ...

mymodule.urllib.urlopen = dummy_urlopen

Then, when your tests invoke functions in mymodule, dummy_urlopen() will be called instead of the real urlopen(). Dynamic languages like Python make it super easy to stub out methods and classes for testing.

See my blog posts at http://softwarecorner.wordpress.com/ for more information about stubbing out dependencies for tests.

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