How to use mocks in Python?

Question

I know that there are tutorials and other posts about mocks in Python but I'm new to testing and mocks, I read some posts and watched a tutorial but they confused me more. I have two examples that I just wrote fast and I wanna know how can I use mocks to test two examples.

In the first example test the the value that gets returned and in the second one test that a file gets created if we create a new instance of the 'MyFile' class.

1:

def get_link_tags(url):
    response = requests.get(url)

    pattern = re.compile("<link.*>")
    found_link_tags = pattern.findall(response.text)
    return found_link_tags

2:

class MyFile:
    def __init__(self, filename, content):
        self.content = content

        with open(filename, "w") as fl:
            fl.write(content)

Thank you

chepner · Accepted Answer · 2016-05-29 22:26:05Z

3

In the first example, you want to mock requests.get so that you can control its return value. Since requests.get is actually a Mock object when you call get_link_tags, its return value will be as well, so you can configure its text attribute to be whatever data you want.

with mock.patch('requests.get') as mock_get:
    mock_get.return_value.text = "my test data"
    rv = get_link_tags("http://example.com")

The second example is a little tricker; the mock library provides a function to make it simpler.

m = mock.mock_open()
with mock.patch('__main__.open', m):
    f = MyFile('foo.txt', 'data')
    h = m()
    h.write.assert_called_once_with('data')

m is a mock object that replaces open everywhere in the body of the with statement. The key thing to note is that when m is called both inside MyFile and manually in the next line, a reference to the same mock object is returned. This lets you test how the fake file inside MyFile is used.

answered May 29, 2016 at 22:26

chepner

538k77 gold badges594 silver badges746 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

kushtrimh Over a year ago

Since there are not any <link> tags in response.text (which now is the string "my test data") the get_link_tags will return an empty list. Is self.assertEqual(rv, []) appropriate to complete the test or should any of the mock assert methods be called?

chepner Over a year ago

In this case, self.assertEqual is fine, since you aren't concerned with the actual behavior of the mock so much as what the rest of get_link_tags does with the data the mock returns. Ideally, you would have multiple tests, each one setting mock_get.return_value.text to different values, to confirm that get_link_tags does the right thing with each.

chepner Over a year ago

One thing you might do with the mock object, for example, is verify that it is called with the argument passed to get_link_tags.

kushtrimh Over a year ago

Thank you so much for your help.

Collectives™ on Stack Overflow

How to use mocks in Python?

1:

2:

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1:

2:

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related