How to mock class used in separate namespace?

Question

Package Structure:

pqr/pq.py
test.py

pqr/pq.py has following structure

Where lmn is globally installed pip module

Structure of pq.py

from lmn import Lm

class Ab():

    def __init__(self):
        self.lm = Lm()

    def echo(self, msg):
        print msg

test.py has following structure

from pqr.pq import Ab

How to mock Lm() class over here so to test all method from Ab class?

Answer 1

It doesn't really matter where Lm came from. You imported Lm into the pqr.pq namespace as a global there, so you'd only have replace that name there and nowhere else. That's because the Ab.__init__ method will look for it 'locally' in it's own module.

So using the mock library all you need to do is patch the name pqr.pq.Lm:

import mock
from pqr.pq import Ab

with mock.patch('pqr.pq.Lm') as mock_lm:
    # within this block, `Lm` in the `pqr.pq` namespace has been replaced
    # by a mock object
    test_instance = Ab()
    # assert that the patch succeeded; .return_value is used because
    # Ab.__init__ *calls* Lm()
    assert test_instance.lm is mock_lm.return_value

Also see the Where to patch section of the mock documentation.