Question

I have a function call that returns an object:

r = Foo(x,y)

where r has a rich set of nested properties. For example, I can access r.prop_a.prop_b.prop_c. I would like to mock Foo, such that a specific leaf property of r is modified, i.e. such that r.prop_a.prop_b.prop_c returns a value under my control:

>> r = Foo(x,y)
>> r.prop_a.prop_b.prop_c
'fish'
>> # some mock magic patching of Foo is taking place here
>> r = Foo(x,y)
>> r.prop_a.prop_b.prop_c
'my_fish'

I do not care about intermediate properties much.

Is there an elegant way to mock nested properties with mock?

Answer 1

Replace the mock object’s attribute call as you would expect:

>> r1 = r_original(x, y)
>> r1.prop_a.prop_b.prop_c
'fish'

>> returner = mock.MagicMock()
>> returner.prop_a.prop_b.prop_c = 'fish'
>> r_mocked = mock.MagicMock(spec_set=r_original, return_value=returner)
>> r2 = r_mocked(x, y)
>> r2.prop_a.prop_b
MagicMock name='returner.prop_a.prop_b' id='87412560'>
>> r2.prop_a.prop_b.prop_c
'fish'

This allows you the full power of mocking while defining a specific value.