Python unit testing functions by using mocks

Question

I am fairly new to python development and I am not sure what will be the best way to inject mocks into a function for unit testing.

I have a function which looks like:

import exampleModule

def func():
    ls = createList()
    exampleModule.send(ls)

In the above code I want to mock exampleModule.send method.

Should I pass the method as argument to the function? Like:

def func(invokeMethod):
    ls = createList()
    invokeMethod(ls)

And in unit test I can pass the mock. But I do not want the caller to specify the invocation method.

What is the right way of doing it?

Maxim · Accepted Answer · 2012-11-13 19:41:17Z

2

You can use mock library by Michael Foord, which is part Python 3. It makes this kind of mocking very convenient. One way of doing it would be:

>>> from mock import patch
>>> import exampleModule
>>>    
>>> def func():
...     ls = []
...     exampleModule.send(ls)
... 
>>> with patch('exampleModule.send') as send:
...     func()
...     assert send.called

Here we use it as a context manager. But you ca also use patch as a decorator. But there are more ways of using mock and it will probably meet all your needs in mocking/stubbing.

answered Nov 13, 2012 at 19:41

Maxim

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Community · Accepted Answer · 2017-05-23 11:43:09Z

1

Python supports functions as first class citizens, so you can override the implementation of a method for unit testing purposes.

This approach basically shows you the way.

class Foo
   def thing_to_mock():
      really_expensive_stuff()

   def thing_to_test():
       i = 1 + 2
       thing_to_mock()
       return i

class FooTest
  def testingThingToTest():
       def mocker():
           pass
       toTest = Foo()
       toTest.thing_to_mock = mocker
       # assert here

Alternately, in Python 3.3 you can use the built-in mocking support.

edited May 23, 2017 at 11:43

CommunityBot

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answered Nov 13, 2012 at 19:15

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1 Comment

Prasanna Over a year ago

The difference is, in my case, I am not testing a method (function in a class) but a high level function.

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