Test function with lru_cache decorator

Question

I'm attempting to test a a method that is memoized through lru_cache (since it's an expensive database call). with pytest-mock.

A simplified version of the code is:

class User:

    def __init__(self, file):
        # load a file

    @lru_cache
    def get(self, user_id):
        # do expensive call

Then I'm testing:

class TestUser:

    def test_get_is_called(self, mocker):
        data = mocker.ANY
        user = User(data)
        repository.get(user_id)
        open_mock = mocker.patch('builtins.open', mocker.mock_open())
        open_mock.assert_called_with('/foo')

But I'm getting the following error:

TypeError: unhashable type: '_ANY'

This happens because functools.lru_cache needs the keys stored to be hashable i.e. have a method __hash__ or __cmp__ implemented.

How can I mock such methods in a mocker to make it work?

I've tried

user.__hash__.return_value = 'foo'

with no luck.

Answer 1

Instead of using mocker.ANY (an object which is intented to be used in assertions as a placeholder that's equal to any object) I believe you instead want to use a sentinel object (such as mocker.sentinel.DATA).

This appears to work from a quick test:

from functools import lru_cache

@lru_cache(maxsize=None)
def f(x):
    return (x, x)


def test(mocker):
    ret = f(mocker.sentinel.DATA)
    assert ret == (mocker.sentinel.DATA, mocker.sentinel.DATA)