unittest.mock: asserting partial match for method argument

Question

Rubyist writing Python here. I've got some code that looks kinda like this:

result = database.Query('complicated sql with an id: %s' % id)

database.Query is mocked out, and I want to test that the ID gets injected in correctly without hardcoding the entire SQL statement into my test. In Ruby/RR, I would have done this:

mock(database).query(/#{id}/)

But I can't see a way to set up a 'selective mock' like that in unittest.mock, at least without some hairy side_effect logic. So I tried using the regexp in the assertion instead:

with patch(database) as MockDatabase:
  instance = MockDatabase.return_value
  ...
  instance.Query.assert_called_once_with(re.compile("%s" % id))

But that doesn't work either. This approach does work, but it's ugly:

with patch(database) as MockDatabase:
  instance = MockDatabase.return_value
  ...
  self.assertIn(id, instance.Query.call_args[0][0])

Better ideas?

Answer 1

import mock

class AnyStringWith(str):
    def __eq__(self, other):
        return self in other

...
result = database.Query('complicated sql with an id: %s' % id)
database.Query.assert_called_once_with(AnyStringWith(id))
...

Preemptively requires a matching string

def arg_should_contain(x):
    def wrapper(arg):
        assert str(x) in arg, "'%s' does not contain '%s'" % (arg, x)
    return wrapper

...
database.Query = arg_should_contain(id)
result = database.Query('complicated sql with an id: %s' % id)

UPDATE

Using libraries like callee, you don't need to implement AnyStringWith.

from callee import Contains

database.Query.assert_called_once_with(Contains(id))

https://callee.readthedocs.io/en/latest/reference/operators.html#callee.operators.Contains