Monday, 20 December 2010

Mocking empty collections with FalseMock

A friend of mine described to me a PITA he had with mock - it doesn't play well with a common Python idiom:

if collection:
    for element in collection:
What he expected as default behaviour was for the mock to be iteratable as an empty collection. Instead, he got:
>>> m = mock.Mock()
>>> for x in m:
...     print x
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'Mock' object is not iterable
As it turns out, there is a simple solution to this problem: all you need to do is to implement the magic method yourself.
>>> class FalseMock(mock.Mock):
...     def __nonzero__(self):
...             return False
This behaves as you would expect: It evaluates to False.
>>> m = FalseMock()
>>> bool(m)
It still works as every other Mock object would:
>>> m.a.b.c.d.return_value = "hi"
>>> m.a.b.c.d()
It keeps its behaviour in all Mocks generated as a result of getting an attribute
>>> m.a.b.c.d


flynn said...
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Piotr Czapla said...

You can also use MagicMock that implements the magic methods. The only minor problem is that it interface is not consistent with list interface.

Namely bool(MagicMock()) == True while list(MagicMock()) == []