CODEWITHSUNDEEP
pythonpytest
MaverickD
MaverickD

Reputation: 1657

pytest mock os.listdir to return empty list

I have a function in my program that list all the files in a given path and I am trying to write a test that passes when there are no files exists in the path provided (empty output i.e []) I am learning about mocker fixture of pytest to do it. Here is what i have written,

def test_no_dirs(mocker):
    mocker.patch('os.listdir')
    assert get_list() #get_list returns ['abc.json', 'test.json', 'test2.json']
    os.listdir.assert_called_with('/etc/app_data/',stdout=[])

I have used mocker as a parameter first then patch the os.listdir function. os.listdir is called in get_list(), but i don't know how to change the return value of os.listdir to empty list, [] to mock empty directory.

When I run above command I get the following error,

E       AssertionError: Expected call: listdir('/etc/app_data/', stdout='[]')
E       Actual call: listdir('/etc/app_data/')
E
E       pytest introspection follows:
E
E       Kwargs:
E       assert {} == {'stdout': '[]'}
E         Right contains more items:
E         {'stdout': '[]'}
E         Full diff:
E         - {}
E         + {'stdout': '[]'}

How can I mock os.listdir to return empty value and pass the test?

If i remove stdout=[], the test PASS but its not really doing what i want to do, which is to pass when there are no files.

That's the code for get_list()

import os
def get_list():
    return os.listdir('/etc/app_data/')

Upvotes: 2

Views: 1685

Answers (1)

wim
wim

Reputation: 363556

The pytest-mock version should look something like this:

def test_no_dirs(mocker):
    mock = mocker.patch('os.listdir', return_value=[])
    result = get_list()
    assert result == []  # because that's the `return_value` mocked
    mock.assert_called_once_with('/etc/app_data/')

Note that users who do from os import listdir will need to mock in a different namespace!

Upvotes: 3

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