Mocking out methods on any instance of a python class

Question

I want to mock out methods on any instance of some class in the production code in order to facilitate testing. Is there any library in Python which could facilitate this?

Answer 1

Mock is the way to do it, alright. It can be a bit tricky to make sure you're patching the instance method on any instances created from the class.

# a.py
class A(object):
    def foo(self):
        return "A's foo"

# b.py
from a import A

class B(object):
    def bar(self):
        x = A()
        return x.foo()

# test.py
from a import A
from b import B
import mock

mocked_a_class = mock.Mock()
mocked_a_instance = mocked_a_class.return_value
mocked_a_instance.foo.return_value = 'New foo'

with mock.patch('b.A', mocked_a_class):  # Note b.A not a.A
    y = B()
    if y.bar() == "New foo":
        print "Success!"

Referenced in the docs, at the para starting "To configure return values on methods of instances on the patched class..."