Mock for line in open(file):

Question

I am wanting to unittest a component of my application. The code looks a little like below.

def read_content_generator(myfile):
    for line in open(myfile):
        # do some string manipulation.
        yield result

The problem I am having is that I cannot mock the open() functionality within a for loop.

What I am aiming for is a unittest like this: (I know this code is not right but its just an example of what I am trying to do):

def test_openiteration(self):
    with mock.patch('open') as my_openmock:
        my_openmock.return_value = ['1','2','3']
        response = myfunction()
        self.assertEquals([1,2,3], response)

I'm really not sure what the goal is here. Can you clarify? – Veedrac Jun 13 '14 at 10:01 — Veedrac, Jun 13 '14 at 10:01
@veedrac Apologies I have now reworded – user204088 Jun 13 '14 at 10:23 — user204088, Jun 13 '14 at 10:23

score 10 · Answer 1 · edited May 23 '17 at 12:18

You can mock open() to return StringIO object.

mymodule.py:

def read_content_generator(myfile):
    with open(myfile) as f:
        for line in f:
            yield '<{}>'.format(line)

Note that I've used with statement there.

test_mymodule.py:

import io
import unittest
import unittest.mock as mock

import mymodule


class Tests(unittest.TestCase):
    def test_gen(self):
        fake_file = io.StringIO('foo\nbar\n')
        with mock.patch('mymodule.open', return_value=fake_file, create=True):
            result = list(mymodule.read_content_generator('filename'))
        self.assertEqual(result, ['<foo\n>' , '<bar\n>'])

Works for python3.4.

At first I tried to use mock.mock_open(read_data='1\n2\n3\n') but iteration support seems to be broken.

Using python 2.7.13, I had to use `StringIO.StringIO` instead of `io.StringIO` — Jeff, May 26 '17 at 17:51

score 1 · Answer 2 · answered Jun 13 '14 at 10:29

1

There are two easy options available:

Change read_content_generator to take a file, not a file name, and mock that with an io.StringIO.
Make a temporary file. There are good modules for this.

answered Jun 13 '14 at 10:29

Veedrac

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tempfile works. https://docs.python.org/3/library/tempfile.html#tempfile-examples It's also possible that your function works with a StringIO object. – IceArdor Mar 21 '17 at 04:00

Mock for line in open(file):

2 Answers2

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