Mocking python function based on input arguments

Side effect takes a function (which can also be a lambda function), so for simple cases you may use:

m = MagicMock(side_effect=(lambda x: x+1))

As indicated at Python Mock object with method called multiple times

A solution is to write my own side_effect

def my_side_effect(*args, **kwargs):
    if args[0] == 42:
        return "Called with 42"
    elif args[0] == 43:
        return "Called with 43"
    elif kwargs['foo'] == 7:
        return "Foo is seven"

mockobj.mockmethod.side_effect = my_side_effect

That does the trick

If you "want to return a fixed value when the input parameter has a particular value", maybe you don't even need a mock and could use a dict along with its get method:

foo = {'input1': 'value1', 'input2': 'value2'}.get

foo('input1')  # value1
foo('input2')  # value2

This works well when your fake's output is a mapping of input. When it's a function of input I'd suggest using side_effect as per Amber's answer.

You can also use a combination of both if you want to preserve Mock's capabilities (assert_called_once, call_count etc):

self.mock.side_effect = {'input1': 'value1', 'input2': 'value2'}.get

If side_effect_func is a function then whatever that function returns is what calls to the mock return. The side_effect_func function is called with the same arguments as the mock. This allows you to vary the return value of the call dynamically, based on the input:

>>> def side_effect_func(value):
...     return value + 1
...
>>> m = MagicMock(side_effect=side_effect_func)
>>> m(1)
2
>>> m(2)
3
>>> m.mock_calls
[call(1), call(2)]

http://www.voidspace.org.uk/python/mock/mock.html#calling