Mocking out methods on any instance of a python class

Needing to mock out methods when testing is very common and there are lots of tools to help you with it in Python. The danger with “monkey patching” classes like this is that if you don’t undo it afterwards then the class has been modified for all other uses throughout your tests.

My library mock, which is one of the most popular Python mocking libraries, includes a helper called “patch” that helps you to safely patch methods or attributes on objects and classes during your tests.

The mock module is available from:

http://pypi.python.org/pypi/mock

The patch decorator can be used as a context manager or as a test decorator. You can either use it to patch out with functions yourself, or use it to automatically patch with Mock objects that are very configurable.

from a import A
from b import B

from mock import patch

def new_foo(self):
    return "New foo"

with patch.object(A, 'foo', new_foo):
    y = B()
    if y.bar() == "New foo":
        print "Success!"

This handles the unpatching for you automatically. You could get away without defining the mock function yourself:

from mock import patch

with patch.object(A, 'foo') as mock_foo:
    mock_foo.return_value = "New Foo"

    y = B()
    if y.bar() == "New foo":
        print "Success!"