Behavior of mock, patch, pytest mocker, examples

test_patch.py

import contextlib
import pytest
from unittest.mock import patch


class A(object):
    def say(self, msg):
        return f'A say {msg!r}.'

    @contextlib.contextmanager
    def start(self, msg):
        print(f'A start --- {msg!r}')
        yield f'yield from A - {msg!r}'
        print(f'B end   --- {msg!r}')


def test_say():
    assert A().say('hello') == "A say 'hello'."


def test_patched_say1(mocker):
    with patch.object(A, 'say', return_value='hogehoge') as mock_say:
        a = A()
        assert a.say('hello') == "hogehoge"
    mock_say.assert_called_once_with('hello')


def test_patched_say2(mocker):
    def ret_fugafuga(*args):
        return 'fugafuga'

    with patch.object(A, 'say', wraps=ret_fugafuga) as mock:
        print(mock)
        assert A().say('hello') == "fugafuga"
        mock.assert_called_once_with('hello')


def test_patched_say3(mocker):
    a_say = A.say

    def wrapped_say(self, *args):
        return f'Wrap say - {a_say(self, *args)}'

    mocker.patch.object(A, 'say', new=wrapped_say)
    assert A().say('hello') == "Wrap say - A say 'hello'."


def test_patched_say4():
    with patch.object(A, 'say') as mock:
        mock.return_value = 'hogehoge'
        assert A().say('hello') == "hogehoge"
    mock.assert_called_once_with('hello')


def test_patched_say5(mocker):
    # Using mocker.patch as context manager is not supported
    # https://pypi.org/project/pytest-mock/
    with mocker.patch.object(A, 'say') as mock:
        mock.return_value = 'hogehoge'
        with pytest.raises(AssertionError):
            assert A().say('hello') == "hogehoge"  # Doesn't work

    with pytest.raises(AssertionError):
        mock.assert_called_once_with('hello')  # Doesn't work


def test_start():
    with A().start('hello') as msg:
        assert msg == "yield from A - 'hello'"


def test_patch_start1():
    with patch.object(A, 'start') as mock_start:
        mock_start.return_value.__enter__.return_value = 'yield from mock'
        with A().start('hello') as msg:
            assert msg == 'yield from mock'


def test_patch_start2():
    a_start = A.start

    @contextlib.contextmanager
    def wrap_start(self, msg):
        print(f'wrap_start start - {msg}')
        with a_start(self, msg) as orig_msg:
            yield f'yield from wrap_start  - {orig_msg!r}'
        print(f'wrap_start end   - {msg}')

    with patch.object(A, 'start', wrap_start) as mock_start:
        with A().start('hello') as msg:
            assert msg == "yield from wrap_start  - \"yield from A - 'hello'\""