unit testing - Python Mock object with method called multiple times

Question

I have a class that I'm testing which has as a dependency another class (an instance of which gets passed to the CUT's init method). I want to mock out this class using the Python Mock library.

What I have is something like:

mockobj = Mock(spec=MyDependencyClass)
mockobj.methodfromdepclass.return_value = "the value I want the mock to return"
assertTrue(mockobj.methodfromdepclass(42), "the value I want the mock to return")

cutobj = ClassUnderTest(mockobj)

Which is fine, but "methodfromdepclass" is a parameterized method, and as such I want to create a single mock object where depending on what arguments are passed to methodfromdepclass it returns different values.

The reason I want this parameterized behaviour is I want to create multiple instances of ClassUnderTest that contain different values (the values of which are produced by what gets returned from the mockobj).

Kinda what I'm thinking (this of course does not work):

mockobj = Mock(spec=MyDependencyClass)
mockobj.methodfromdepclass.ifcalledwith(42).return_value = "you called me with arg 42"
mockobj.methodfromdepclass.ifcalledwith(99).return_value = "you called me with arg 99"

assertTrue(mockobj.methodfromdepclass(42), "you called me with arg 42")
assertTrue(mockobj.methodfromdepclass(99), "you called me with arg 99")

cutinst1 = ClassUnderTest(mockobj, 42)
cutinst2 = ClassUnderTest(mockobj, 99)

# now cutinst1 & cutinst2 contain different values

How do I achieve this "ifcalledwith" kind of semantics?

Answer 1

Try 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