Pytest mocking a Content Manager used by for loop

Question

I have a script that checks if a line in a given file starts with a key word and returns the rest of the line if so:

class Check():
    def check_file(file, varible):
        file.seek(0)
        for line in file:
            if varible.strip() in line.strip():
                return line.strip()[len(varible):].strip()

Now I want to write a test it. I found this post suggesting to use mock_open and patch: How do I mock an open used in a with statement (using the Mock framework in Python)?

Somewhere else I found the suggesting to put multiple Mocks in a list and use that. I tried both:

import pytest
from unittest.mock import patch, mock_open, Mock

from check_file import Check

def test_check_file():
        with patch("builtins.open", mock_open(read_data="flying flying")) as mock_file:

            with open("some_file", "r") as file:
                print(Check.check_file(file, "flying"))
                assert Check.check_file(file, "flying") == "flying"

def test_check_file2():
    my_mock1 = Mock()
    my_mock1.return_value =  "monthy monthy"
    my_mock2 = Mock()
    my_mock2.return_value = "python's python's"
    my_mock3 = Mock()
    my_mock3.return_value = "flying flying"
    my_mock4 = Mock()
    my_mock4.return_value = "circus circus"

    my_mock = [ my_mock1, my_mock2, my_mock3, my_mock4 ]
    print(Check.check_file(my_mock, "flying"))

And got this error messages:

test_check_file.py::test_check_file None
FAILED
test_check_file.py::test_check_file2 FAILED

====================== FAILURES ========================
______________________ test_check_file _______________________

    def test_check_file():
            with patch("builtins.open", mock_open(read_data="flying flying")) as mock_file:

                with open("some_file", "r") as file:
                    print(Check.check_file(file, "flying"))
>                   assert Check.check_file(file, "flying") == "flying"
E                   AssertionError: assert None == 'flying'
E                     -None
E                     +'flying'

test_check_file.py:11: AssertionError
________________________ test_check_file2 _________________________

    def test_check_file2():
        my_mock1 = Mock()
        my_mock1.return_value =  "monthy monthy"
        my_mock2 = Mock()
        my_mock2.return_value = "python's python's"
        my_mock3 = Mock()
        my_mock3.return_value = "flying flying"
        my_mock4 = Mock()
        my_mock4.return_value = "circus circus"

        my_mock = [ my_mock1, my_mock2, my_mock3, my_mock4 ]
>       print(Check.check_file(my_mock, "flying"))

test_check_file.py:24:
_ _ _ _ _ _ _ _ _  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

file = [<Mock id='140386010077000'>, <Mock id='140386010077056'>, <Mock id='140386010077224'>, <Mock id='140386010077112'>], varible = 'flying'

    def check_file(file, varible):
>       file.seek(0)
E       AttributeError: 'list' object has no attribute 'seek'

check_file.py:4: AttributeError
====================== 2 failed in 0.54 seconds =========================

Is there a way that mock_open does not produce None values if called in a loop?

The approach with the list full of Mock object may have worked if I did not need seek(0).

Pytest mocking a Content Manager used by for loop

0 Answers0