Python unittest mock: Is it possible to mock the value of a method's default arguments at test time?

Question

I have a method that accepts default arguments:

def build_url(endpoint, host=settings.DEFAULT_HOST):
    return \'{}{}\'.format(host, endpoint)

Answer 1

Functions store their parameter default values in the func_defaults attribute when the function is defined, so you can patch that. Something like

def test_build_url(self):
    """ If only endpoint is supplied should default to settings"""

    # Use `func_defaults` in Python2.x and `__defaults__` in Python3.x.
    with patch.object(build_url, 'func_defaults', ('domain',)):
      result = build_url('/end')
      expected = 'domain/end'

    self.assertEqual(result,expected)

I use patch.object as a context manager rather than a decorator to avoid the unnecessary patch object being passed as an argument to test_build_url.