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On Mocking in Python
The Mock library
Michael Foord (who used to work for the company that spawned PythonAnywhere,
just before I joined) wrote the excellent “Mock” library that’s now been integrated
into the standard library of Python 3. It contains most everything you might need
for mocking in Python.
The patch decorator
unittest.mock
provides a function called
patch
, which can be used to “mock out”
any object from the module you’re testing. It’s commonly used as a decorator on a
test method, or even at the class level, where it’s applied to all the test methods of
that class.
Mocks are truthy and can mask errors
Be aware that mocking things out can cause counterintuitive behaviour in
if
state‐
ments. Mocks are truthy, and they can also mask errors, because they have all at‐
tributes and methods.
Too many mocks are a code smell
Overlymocky tests end up very tightly coupled to their implementation. Sometimes
this is unavoidable. But, in general, try to find ways of organising your code so that
you don’t need too many mocks.
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