In python, how to write unit test for a function is using with open file?

You can use unittest.mock.mock_open(mock=None, read_data=None) to mock the open function.

E.g.

main.py:

class MyClass:
    def get_version(self):
        version = ''
        with open("file_path") as openfile:
            for line in openfile:
                sline = line.split()
                for row, column in enumerate(sline):
                    if column == "version=":
                        version = sline[row+1].strip('"')

        return version

test_main.py:

from main import MyClass
import unittest
from unittest.mock import mock_open, patch


class TestMain(unittest.TestCase):
    def test_get_version(self):
        m = mock_open(read_data='version= 1.0.0')
        with patch('builtins.open', m) as mocked_open:
            myclass_instace = MyClass()
            version = myclass_instace.get_version()
            self.assertEqual(version, '1.0.0')
            m.assert_called_with('file_path')


if __name__ == '__main__':
    unittest.main()

unit test results with 100% coverage:

.
----------------------------------------------------------------------
Ran 1 test in 0.011s

OK
Name                                      Stmts   Miss  Cover   Missing
-----------------------------------------------------------------------
src/stackoverflow/60183706/main.py           10      0   100%
src/stackoverflow/60183706/test_main.py      14      0   100%
-----------------------------------------------------------------------
TOTAL                                        24      0   100%

Python version: Python 3.7.5

You can use mock_open helper function:

unittest.mock.mock_open(mock=None, read_data=None)

A helper function to create a mock to replace the use of open(). It works for open() called directly or used as a context manager.

Previous sentences come from the documentation of the module unittest.mock

Because the production method get_version() reads a file by a for loop you have to define methods __iter__ and __next__ for the mock object instantiated by mock_open as suggested by this post.

So if your production code is stored in the file my_class.py as below:

class MyClass:
    def get_version(self):
        version = ''
        with open("file_path") as openfile:
            for line in openfile:
                sline = line.split()
                for row, column in enumerate(sline):
                    if column == "version=":
                        version = sline[row+1].strip('"')
        return version

a possible test which uses the helper function mock_open could be the following:

from my_class import MyClass
import unittest
from unittest.mock import mock_open, patch

class TestMyClass(unittest.TestCase):
    def test_get_version(self):
        # by the argument read_data set the content of the file
        mocked_open = mock_open(read_data='version= 2.1\nsecond line\n')
        EXPECTED_VERSION = "2.1"
        mocked_open_instance = mocked_open.return_value
        mocked_open_instance.__iter__ = lambda self: self
        mocked_open_instance.__next__ = lambda self: next(iter(self.readline, ''))
        with patch('builtins.open', mocked_open):
            sut = MyClass()
            version = sut.get_version()
            self.assertEqual(EXPECTED_VERSION, version)
            mocked_open.assert_called_with('file_path')
            # for the file content 'version= 2.1\nsecond line\n' the readline function must be called 3 times
            self.assertEqual(3, mocked_open_instance.readline.call_count)

if __name__ == '__main__':
    unittest.main()

Version of Python used is: Python 3.6.