Recipes

Classes as factories

We want to test the following code:

import requests

def fetch(url):
    session = requests.Session()
    return session.get(url)

In a traditional sense this code is not designed for testability. But we don’t care here.

Python has no new keyword to get fresh instances from classes. Man, that was a good decision, Guido! So the uppercase S in requests.Session() doesn’t have to stop us in any way. It looks like a function call, and we treat it like such: The plan is to replace Session with a factory function that returns a (mocked) session:

from mockito import when, mock, verifyStubbedInvocationsAreUsed

def test_fetch(unstub):
    url = 'http://example.com/'
    response = mock({'text': 'Ok'}, spec=requests.Response)
    # remember: `mock` here just creates an empty object specced after
    #           requests.Session
    session = mock(requests.Session)
    # `when` here configures the mock
    when(session).get(url).thenReturn(response)
    # `when` *patches* the globally available *requests* module
    when(requests).Session().thenReturn(session)  # <=

    res = fetch(url)
    assert res.text == 'Ok'

    # no need to verify anything here, if we get the expected response
    # back, `url` must have been passed through the system, otherwise
    # mockito would have thrown.
    # We *could* ensure that our mocks are actually used, if we want:
    verifyStubbedInvocationsAreUsed()

Faking magic methods

We want to test the following code:

import requests

def fetch_2(url):
    with requests.Session() as session:
        return session.get(url)

It’s basically the same problem, but we need to add support for the context manager, the with interface:

from mockito import when, mock, args

def test_fetch_with(unstub):
    url = 'http://example.com/'
    response = mock({'text': 'Ok'}, spec=requests.Response)

    session = mock(requests.Session)
    when(session).get(url).thenReturn(response)
    when(session).__enter__().thenReturn(session)  # <=
    when(session).__exit__(*args)                  # <=

    when(requests).Session().thenReturn(session)

    res = fetch_2(url)
    assert res.text == 'Ok'