I watched Raymond Hettinger's Pycon talk "Super Considered Super" and learned a little bit about Python's MRO (Method Resolution Order) which linearises a classes "parent" classes in a deterministic way. We can use this to our advantage, like in the below code, to do dependency injection. So now, naturally, I want to use super
for everything!
In the example below, the User
class declares it's dependencies by inheriting from both LoggingService
and UserService
. This isn't particularly special. The interesting part is that we can use the Method Resolution Order also mock out dependencies during unit testing. The code below creates a MockUserService
which inherits from UserService
and provides an implementation of the methods we want to mock. In the example below, we provide an implementation of validate_credentials
. In order to have MockUserService
handle any calls to validate_credentials
we need to position it before UserService
in the MRO. This done by creating a wrapper class around User
called MockUser
and having it inherit from User
and MockUserService
.
Now, when we do MockUser.authenticate
and it, in turn, calls to super().validate_credentials()
MockUserService
is before UserService
in the Method Resolution Order and, since it offers a concrete implementation of validate_credentials
this implementation will be used. Yay – we've successfully mocked out UserService
in our unit tests. Consider that UserService
might do some expensive network or database calls – we've just removed the latency factor of this. There is also no risk of UserService
touching live/prod data.
class LoggingService(object):
"""
Just a contrived logging class for demonstration purposes
"""
def log_error(self, error):
pass
class UserService(object):
"""
Provide a method to authenticate the user by performing some expensive DB or network operation.
"""
def validate_credentials(self, username, password):
print('> UserService::validate_credentials')
return username == 'iainjames88' and password == 'secret'
class User(LoggingService, UserService):
"""
A User model class for demonstration purposes. In production, this code authenticates user credentials by calling
super().validate_credentials and having the MRO resolve which class should handle this call.
"""
def __init__(self, username, password):
self.username = username
self.password = password
def authenticate(self):
if super().validate_credentials(self.username, self.password):
return True
super().log_error('Incorrect username/password combination')
return False
class MockUserService(UserService):
"""
Provide an implementation for validate_credentials() method. Now, calls from super() stop here when part of MRO.
"""
def validate_credentials(self, username, password):
print('> MockUserService::validate_credentials')
return True
class MockUser(User, MockUserService):
"""
A wrapper class around User to change it's MRO so that MockUserService is injected before UserService.
"""
pass
if __name__ == '__main__':
# Normal useage of the User class which uses UserService to resolve super().validate_credentials() calls.
user = User('iainjames88', 'secret')
print(user.authenticate())
# Use the wrapper class MockUser which positions the MockUserService before UserService in the MRO. Since the class
# MockUserService provides an implementation for validate_credentials() calls to super().validate_credentials() from
# MockUser class will be resolved by MockUserService and not passed to the next in line.
mock_user = MockUser('iainjames88', 'secret')
print(mock_user.authenticate())
This feels quite clever, but is this a good and valid use of Python's multiple inheritance and Method Resolution Order? When I think about inheritance in the way that I learned OOP with Java this feels completely wrong because we can't say User
is a UserService
or User
is a LoggingService
. Thinking that way, using inheritance the way the above code uses it doesn't make much sense. Or is it? If we use inheritance purely just to provide code reuse, and not thinking in terms of parent->children relationships, then this doesn't seem so bad.
Am I doing it wrong?
Best Answer
No. This is a theoretical intended usage of the C3 linearization algorithm. This goes against your familiar is-a relationships, but some consider composition to be preferred to inheritance. In this case, you composed some has-a relationships. It seems you're on the right track (though Python has a logging module, so the semantics are a bit questionable, but as an academic exercise it's perfectly fine).
I don't think mocking or monkey-patching is a bad thing, but if you can avoid them with this method, good for you - with admittedly more complexity, you have avoided modifying the production class definitions.
It looks good. You have overridden a potentially expensive method, without monkey-patching or using a mock patch, which, again, means you haven't even directly modified the production class definitions.
If the intent was to exercise the functionality without actually having credentials in the test, you should probably do something like:
instead of using your real credentials, and check that the parameters are received correctly, perhaps with assertions (as this is test code, after all.):
Otherwise, looks like you've figured it out. You can verify the MRO like this:
And you can verify that the
MockUserService
has precedence over theUserService
.