Python unit test that uses an external data file

Question

I have a Python project that I\'m working on in Eclipse and I have the following file structure:

/Project
    /projectname
        module1.py
        module2.py          


        

Answer 1

Unit test that access the file system are generally not a good idea. This is because the test should be self contained, by making your test data external to the test it's no longer immediately obvious which test the csv file belongs to or even if it's still in use.

A preferable solution is to patch open and make it return a file-like object.

from unittest import TestCase
from unittest.mock import patch, mock_open

from textwrap import dedent

class OpenTest(TestCase):
    DATA = dedent("""
        a,b,c
        x,y,z
        """).strip()

    @patch("builtins.open", mock_open(read_data=DATA))
    def test_open(self):

        # Due to how the patching is done, any module accessing `open' for the 
        # duration of this test get access to a mock instead (not just the test 
        # module).
        with open("filename", "r") as f:
            result = f.read()

        open.assert_called_once_with("filename", "r")
        self.assertEqual(self.DATA, result)
        self.assertEqual("a,b,c\nx,y,z", result)