Python unit test mock, get mocked function’s input arguments

I want a unit test to assert that a variable action within a function is getting set to its expected value, the only time this variable is used is when it is passed in a call to a library.

Class Monolith(object):
    def foo(self, raw_event):
        action =  # ... Parse Event
        # Middle of function
        lib.event.Event(METADATA, action)
        # Continue on to use the build event.

My thought was that I could mock lib.event.Event, and get its input arguments and assert they are of specific value.

>Is this not how mocks work? The mock documentation frustrates me with its inconsistency, half-examples, and plethora of examples that are not related to what I want to do.

Answer

You could use patch decorator and then call assert_called_with to that mocked object like this:

If you have this structure:

example.py
tests.py
lib/__init__.py
lib/event.py

And the content of example.py is:

import lib

METADATA = 'metadata_example'

class Monolith(object):

    def foo(self, raw_event):
        action =  'action_example' # ... Parse Event
        # Middle of function
        lib.event.Event(METADATA, action)
        # Continue on to use the build event.

And the content of lib/event.py is:

class Event(object):

    def __init__(self, metadata, action):
        pass

The code of tests.py should be like:

import mock
import unittest

from lib.event import Event
from example import Monolith


class TestExample(unittest.TestCase):

    @mock.patch('lib.event.Event')
    def test_example1(self, event_mocked):
        # Setup
        m = Monolith()

        # Exercise
        m.foo('raw_event')

        # Verify
        event_mocked.assert_called_with('metadata_example', 'action_example')