Mock with fixture

poem.py

from datetime import datetime

def get_poem_line_for_day():
    """Returns the line of the poem based on the current day of the week."""
    day_of_week = datetime.today().strftime('%A')
    POEM = {
        "Monday": "Monday's child is fair of face",
        "Tuesday": "Tuesday's child is full of grace",
        "Wednesday": "Wednesday's child is full of woe",
        "Thursday": "Thursday's child has far to go",
        "Friday": "Friday's child is loving and giving",
        "Saturday": "Saturday's child works hard for his living",
        "Sunday": "And the child that is born on the Sabbath day is bonny and blithe, and good and gay",
    }
    return POEM.get(day_of_week, "Unknown day")

test_poem.py

from unittest import mock

import pytest

import poem

# setup ----------------------------------------------------------------


POEM = {
        "Monday": "Monday's child is fair of face",
        "Tuesday": "Tuesday's child is full of grace",
        "Wednesday": "Wednesday's child is full of woe",
        "Thursday": "Thursday's child has far to go",
        "Friday": "Friday's child is loving and giving",
        "Saturday": "Saturday's child works hard for his living",
        "Sunday": "And the child that is born on the Sabbath day is bonny and blithe, and good and gay",
    }

# local mock ----------------------------------------------------------

@mock.patch("poem.get_poem_line_for_day")
def test_poem_line_forever_thursday(patched_poem):
    """Uses immediate instantiation"""
    def mock_poem(day, poem=POEM):
        return poem[day]

    patched_poem.return_value = mock_poem(day="Thursday")
    result = poem.get_poem_line_for_day()
    assert result == "Thursday's child has far to go"


# fixture mock ---------------------------------------------------------

@pytest.fixture(scope="function")
def mock_poem_line_factory():
    """Factory function that mocks expected return values."""
    def _get_poem_line(day_name: str, poem: dict = POEM) -> str:
        return poem[day_name]
    
    return _get_poem_line

@mock.patch("poem.get_poem_line_for_day")
def test_poem_line_any_day_we_like(patched_poem, mock_poem_line_factory):
    """Uses deferred instantiation."""
    patched_poem.return_value = mock_poem_line_factory(day_name="Wednesday")
    result = poem.get_poem_line_for_day()
    assert result == "Wednesday's child is full of woe"

    patched_poem.return_value = mock_poem_line_factory(day_name="Friday")
    result = poem.get_poem_line_for_day()
    assert result == "Friday's child is loving and giving"