writing a pytest function to check outputting to a file in python?

Suppose you have this "amazing" piece of software in a file called main.py:

"""
main.py
"""

def write_to_file(text):
    with open("output.txt", "w") as h:
        h.write(text)

if __name__ == "__main__":
    write_to_file("Every great dream begins with a dreamer.")

To test the write_to_file method, you can write something like this in a file in the same folder called test_main.py:

"""
test_main.py
"""
from unittest.mock import patch, mock_open

import main


def test_do_stuff_with_file():
    open_mock = mock_open()
    with patch("main.open", open_mock, create=True):
        main.write_to_file("test-data")

    open_mock.assert_called_with("output.txt", "w")
    open_mock.return_value.write.assert_called_once_with("test-data")

I always try to avoid writing files to disk, even if it's a temporary folder dedicated to my tests: not actually touching the disk makes your tests much faster, especially if you interact with files a lot in your code.

There is the tmpdir fixture which will create you a per-test temporary directory. So a test would look something like this:

def writetoafile(fname):
    with open(fname, 'w') as fp:
        fp.write('Hello\n')

def test_writetofile(tmpdir):
    file = tmpdir.join('output.txt')
    writetoafile(file.strpath)  # or use str(file)
    assert file.read() == 'Hello\n'

Here you're refactoring the code to not be hardcoded either, which is a prime example of how testing your code makes you design it better.