Lately i have been working on implementing OPTIONS (1) verb for the storage nodes in Swift. The proxy server already implements this. The use of OPTIONS is to find out information on the server or other resources. In Swift’s case, OPTIONS is to return the publicly available HTTP methods of the server i.e. allowed methods, and also the server type. For this implementation, a base class was created (this was inherited by all the storage servers – Account, Container, Object) and all the public methods of the server were called by using:

inspect.getmembers(self, predicate=method)

For the returned methods from the server, the attributes ‘publicly_accessible’ is checked and added to a list and the sorted list is returned when OPTIONS is called. But these allowed methods change if the storage node cofig sets the server to be a replication server. To allow this check I extended the base class to include replication server check. The __call__(2) method was making this check earlier. I modified __call__ to access the methods from the base class.

I proceeded to add tests for it and everything went fine – almost. One of the existing Swift tests started failing. It was failing because, __call__ method is being tested by using a Magic Mock(3) method. One of the community members pointed the fact that magic mock method is callable but not a method. __call__ makes replication server check by calling the allowed methods from the base class. And the base class uses the predicate method. So it is modified to use predicate callable.

inspect.getmembers(self, predicate=callable)

That solved the issue for the test and I went on to add further tests to assert the OPTIONS functionality and also the replication server check in __call__ method.

References:

(1)OPTIONS method requests information on communication options available for a specific resource, either at the origin or an intermediary. In Swift’s case, it is at origin which is the proxy server.

(2) __call__ If this method is implemented for a class, it makes a class instance callable. For example:

class foo:
def __init__(self, a, b)
…

def __call__(self, a, b)
…

x = foo(a, b)     -> calls __init__ method
x(a, b)               -> calls __call__ method

(3) Magic Mock: unittest.mock is Python’s test library. MagicMock is a subclass of Mock with default implementations of most of the magic methods. (This is a very interesting concept and it deserves an entire post about it along with examples.)