Mocking a class method that is used via an instance

Question

I'm trying to patch a class method using mock as described in the documentation. The Mock object itself works fine, but its methods don't: For example, their attributes like call_count aren't updated, even though the method_calls attribute of the class Mock object is. More importantly, their return_value attribute is ignored:

class Lib:
 """In my actual program, a module that I import"""
 def method(self):
 return "real"
class User:
 """The class I want to test"""
 def run(self):
 l = Lib()
 return l.method()
with patch("__main__.Lib") as mock:
 #mock.return_value = "bla" # This works
 mock.method.return_value = "mock"
 u = User()
 print(u.run())
>>> 
mock
<MagicMock name='Lib().method()' id='39868624'>

What am I doing wrong here?

EDIT: Passing a class Mock via the constructor doesn't work either, so this is not really related to the patch function.

Answer 1

I have found my error: In order to configure the methods of my mock's instances, I have to use mock().method instead of mock.method.

class Lib:
 """In my actual program, a module that I import"""
 def method(self):
 return "real"
class User:
 """The class I want to test"""
 def run(self):
 l = Lib()
 return l.method()
with patch("__main__.Lib") as mock:
 #mock.return_value = "bla" # This works
 mock().method.return_value = "mock"
 u = User()
 print(u.run())

• patriciasz: I guess you can mark this answer as the best answer. It solved my problem too.
• Michael Ekoka: Your answer confused me for a moment until I understood that subsequent calls to mock() within the context manager would return the same (mock) object each time. I think to make the example a bit less confusing it should use the mock library's intended API as such: mock.return_value.method.return_value = "mock"

Answer 2

from mock import *
class Lib:
 """In my actual program, a module that I import"""
 def method(self):
 return "real"
class User:
 """The class I want to test"""
 def run(self, m):
 return m.method()
with patch("__main__.Lib") as mock:
 #mock.return_value = "bla" # This works
 mock.method.return_value = "mock"
 print User().run(mock)

• Nova: This works but I have to pass my mock to every method that I call. Does this approach have any advantage over my own answer that I'm missing?
• John: Passing a class Mock via the constructor doesn't work either I was still working on the solution, I figured I'd at least share part of what I'd accomplished. Btw nice job figuring it out.

Answer 3

I mock classmethods like this:

def raiser(*args, **kwargs):
 raise forms.ValidationError('foo')
with mock.patch.object(mylib.Commands, 'my_class_method', classmethod(raiser)):
 response=self.admin_client.get(url, data=dict(term='+1000'))