Duck Typing for Mock Objects

Wikipedia defines Duck Typing as:

In computer programming, duck typing is a style of dynamic typing in which an object’s current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class. The name of the concept refers to the duck test, attributed to James Whitcomb Riley, which may be phrased as follows:

“If it walks like a duck and quacks like a duck, I would call it a duck.”

Coming from a strong background in C#, it took awhile for this feature of Python to seem useful. I was use to creating interfaces and creating a proper inheritance chain implementing these interfaces and/or defining abstract base classes, all to have a good set of mock objects to use in unit tests.

I really love the ability to quickly whip out a duck typed class in Python to make simple mock objects so that I am testing my code in a unit test and not boundary objects (e.g. urllib). I don’t need to test whether my request is properly handled by the server at the urllib endpoint and that the response is properly read and processed. I need to instead make sure that my request conforms to a published specification and that the expected result/response from the server is processed properly. This also allows my tests to run offline.

Here is an example of some mock objects that I use in pyphanfare:

# pyphanfare/tests/mocks.py
from pyphanfare.tests.testdata import data

class URLopener:
    def open(self, url):
        params = url.split('?')[1].split('&') 
        for param in params:
            tokens = param.split('=')
            if tokens[0].strip() == 'method':
                return URLhandle(tokens[1].strip())
    def addheader(self, *args):
        self.headers = args

class URLhandle:
    def __init__(self, method):
        self.method = method
    def read(self):
        return data[self.method]

pyphanfare.tests.testdata is just a Python module that contains test data in the form of a dictionary object indexed by method name that I am calling on the API having a value of XML that is expected in the response.