Create a gzip file like object for unit testing

Question

I want to test a Python function that reads a gzip file and extracts something from the file (using pytest).

import gzip

def my_function(file_path):
    output = []

    with gzip.open(file_path, 'rt') as f:
        for line in f:
            output.append('something from line')

    return output

Can I create a gzip file like object that I can pass to my_function? The object should have defined content and should work with gzip.open()

I know that I can create a temporary gzip file in a fixture but this depends on the filesystem and other properties of the environment. Creating a file-like object from code would be more portable.

Answer 1

You can use the io and gzip libraries to create in-memory file objects. Example:

import io, gzip

def inmem():
    stream = io.BytesIO()
    with gzip.open(stream, 'wb') as f:
        f.write(b'spam\neggs\n')
    stream.seek(0)
    return stream

Answer 2

You should never try to test outside code in a unit test. Only test the code you wrote. If you're testing gzip, then gzip is doing something wrong (they should be writing their own unit tests). Instead, do something like this:

from unittest import mock

@mock.Mock('gzip', return_value=b'<whatever you expect to be returned from gzip>')
def test_my_function(mock_gzip):
    file_path = 'testpath'
    output = my_function(file_path=file_path)
    mock_gzip.open.assert_called_with(file_path)
    assert output == b'<whatever you expect to be returned from your method>'

That's your whole unit test. All you want to know is that gzip.open() was called (and you assume it works or else gzip is failing and that's their problem) and that you got back what you expected from the method being tested. You specify what gzip returns based on what you expect it to return, but you don't actually call the function in your test.