Wednesday, June 08, 2005

pre-built mock classes?

There is a (relatively) widely used technique in unit testing, called mock objects. There is even a pMock library which provides a Mock class for a Python environment.


Given the "duck typing" nature of the Python itself, it's pretty trivial to build mocks without using any pre-built libraries. What is less trivial and potentially more worthwhile is to have a library of “stock” mock objects.


For instance, I found myself re-implementing ConnectionStub class again and again for different projects. And there is no shortage of other good candidates as well: socket, web request/response, thread objects, etc. Someone on a Python mailing list asks whether anyone implemented a mock filesystem interface, for example.


Having a library of such mock classes, realizing widely-used and “heavy” interfaces may be a good idea.


The only real problem I can think of is whether it’s possible to make these mocks generic enough. Mocks often contain some (application-specific) hard-coded rules and data to realize desired behavior. For instance, socket mock may fail when user attempts to write a “foobar” string – and unit test will use this to check how code handle these kind of errors. These rules may be generalized but it may lead to mock classes becoming unwieldy.


Even keeping this problem in mind, it’s still a reasonable idea for a library, but obviously I’m biased.


What do you think?


Blogger Kevin Dangoor said...

I think it's a good idea, and a good fit for Python. For many of the basics in the Python std lib, you can make good guesses (or at least flesh out things as you write your own tests) as to what people would like to do with their mocks.

For example, you could make a mock file-like object. Then, you can take over the open builtin to use your mock. With a file-like thing, you'd be interested in providing input data, or perhaps being able to capture the output. That all seems pretty reasonable, and something that more than one person would want!

Since Python is dynamically typed, it's a lot easier to put mocks in place for things, since the mock doesn't have to follow some preexisting inheritance hierarchy.

Some of the more complex mocks (think urllib2, etc.) could be *very* handy!

I'm pretty certain I'd contribute to such a project, but I have too much on my plate to kick it off right now. So, if you get one going, keep us posted!

5:09 PM  
Anonymous Anonymous said...

The following is part of my which I import for every UnitTest.

I typically use NotQuiteNull as mock object and assign WasCalled to the method names the thing undertest needs

sorry about formatting,

class Null:
""" Null objects always and reliably "do nothing" """
def __init__(self, *args, **kwargs): pass
def __call__(self, *args, **kwargs): return self
def __repr__(self): return "Null()"
def __nonzero__(self): return 0
def __str__(self): return ''
def __getattr__(self, name): return self
def __setattr__(self, name, value): return self
def __delattr__(self, name): return self

class NotQuiteNull:
""" NotQuiteNull objects mostly "do nothing" but they can have attributes set """
def __init__(self, *args, **kwargs): pass
def __call__(self, *args, **kwargs): return self
def __repr__(self): return "Null()"
def __nonzero__(self): return 0
def __str__(self): return ''
def __getattr__(self, name): return self
def __delattr__(self, name): return self

class WasCalled:
""" Used to replace of method/func. Remembers if it was called, how often and with which parameters. ""
def __init__(self, returnThis = None):
self.called = 0
self.args = ()
self.kwargs = {}
self.returnThis = returnThis

def __call__(self, *args, **kwargs):
self.called += 1
self.args = args
self.kwargs = kwargs
return self.returnThis

def wasCalled( self ):
return self.called > 0

def wasCalledWith( self, *args, **kwargs):
if not self.wasCalled():
return False
#try to do what they mean
if type(args) == type(list()):
args = tuple(args)
elif type(args) != type(tuple()):
args = (args,)
return args == self.args and kwargs == self.kwargs

def calledWith(self):
return (self.args, self.kwargs)

def clearCalled(self):
self.called = 0
self.args = ()
self.kwargs = {}

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