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

An alternate way to do this: Use functools.partial to provide the "default" args you want. This isn't technically the same thing as overriding them; the call-ee sees an explicit arg, but the call-er doesn't have to provide it. That's close enough most of the time, and it does the Right Thing after the context manager exits:

# mymodule.py
def myfunction(arg=17):
    return arg

# test_mymodule.py
from functools import partial
from mock import patch

import mymodule

class TestMyModule(TestCase):
    def test_myfunc(self):
        patched = partial(mymodule.myfunction, arg=23)
        with patch('mymodule.myfunction', patched):
            self.assertEqual(23, mymodule.myfunction())  # Passes; default overridden
        self.assertEqual(17, mymodule.myfunction()) # Also passes; original default restored

I use this for overriding default config file locations when testing. Credit where due, I got the idea from Danilo Bargen here