OatBran

#--------------------------------------------------
# importme.py
#--------------------------------------------------

def render(arg1):
    print 'real render', arg1
    return arg1


#--------------------------------------------------
# faketest.py
#--------------------------------------------------

from importme import render

def run(somearg):
    return render(somearg)


#--------------------------------------------------
# mintest.py
#--------------------------------------------------

import unittest

class Tester(unittest.TestCase):

    def setUp(self):
        import importme
        self._real_render = importme.render
        importme.render = self._fake_render

        import faketest
        self.run = faketest.run

    def _fake_render(self, arg1):
        # hijack the render function before it actually renders and save arg1
        self.arg1 = arg1
        return self._real_render(arg1)

    def test_A(self):
        self.run('test A')
        self.assertEqual(self.arg1, 'test A')

    def test_B(self):
        self.run('test B')
        self.assertEqual(self.arg1, 'test B')


if __name__ == '__main__':
    unittest.main()


#--------------------------------------------------
# output
#--------------------------------------------------
$ python mintest.py
real render test A
.real render test B
E
======================================================================
ERROR: test_B (__main__.Tester)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "mintest.py", line 24, in test_B
    self.assertEqual(self.arg1, 'test B')
AttributeError: 'Tester' object has no attribute 'arg1'

----------------------------------------------------------------------
Ran 2 tests in 0.000s

FAILED (errors=1)