Using pytest to ensure a file is created and written to

Question

I'm using pytest and want to test that a function writes some content to a file. So I have writer.py which includes:

MY_DIR = '/my/path/'

def my_function():
    with open('{}myfile.txt'.format(MY_DIR), 'w+') as file:
        file.write('Hello')
        file.close()

I want to test /my/path/myfile.txt is created and has the correct content:

import writer

class TestFile(object):

    def setup_method(self, tmpdir):
        self.orig_my_dir = writer.MY_DIR
        writer.MY_DIR = tmpdir

    def teardown_method(self):
        writer.MY_DIR = self.orig_my_dir

    def test_my_function(self):
        writer.my_function()

        # Test the file is created and contains 'Hello'

But I'm stuck with how to do this. Everything I try, such as something like:

        import os
        assert os.path.isfile('{}myfile.txt'.format(writer.MYDIR))

Generates errors which lead me to suspect I'm not understanding or using tmpdir correctly.

How should I test this? (If the rest of how I'm using pytest is also awful, feel free to tell me that too!)

There are two questions that address each part of what you are having difficulty I believe. Check them out and see if they provide any insight. Here is one on testing for writing to the file [here](https://stackoverflow.com/questions/20531072/writing-a-pytest-function-to-check-outputting-to-a-file-in-python) and [here](https://stackoverflow.com/questions/15801662/py-test-how-to-use-a-context-manager-in-a-funcarg-fixture) is one that shows how to use the proper fixture to deal with context managers. — idjaw, Jun 19 '17 at 11:47
You should [mock `open()`](https://stackoverflow.com/questions/1289894/how-do-i-mock-an-open-used-in-a-with-statement-using-the-mock-framework-in-pyth) so you never actually open a file, but just check if it has been used correctly. — Nils Werner, Jun 19 '17 at 11:56
Can we see more details? How do you generate tmpdir? What are errors? — phd, Jun 19 '17 at 15:29
@phd `tmpdir` [is a pytest fixture](https://docs.pytest.org/en/latest/tmpdir.html). I don't like how it appears sort of like 'magic', rather than being imported from somewhere... doesn't feel very explicit. — Phil Gyford, Jun 19 '17 at 15:43

score 3 · Answer 1 · answered Jun 19 '17 at 15:41

I've got a test to work by altering the function I'm testing so that it accepts a path to write to. This makes it easier to test. So writer.py is:

MY_DIR = '/my/path/'

def my_function(my_path):
    # This currently assumes the path to the file exists.
    with open(my_path, 'w+') as file:
        file.write('Hello')

my_function(my_path='{}myfile.txt'.format(MY_DIR))

And the test:

import writer

class TestFile(object):

    def test_my_function(self, tmpdir):

        test_path = tmpdir.join('/a/path/testfile.txt')

        writer.my_function(my_path=test_path)

        assert test_path.read() == 'Hello'

Using pytest to ensure a file is created and written to

1 Answers1