Mock a nested object in Python using Mock()

Question

I have a parent object, which receives a connection object.

I run a method on this connection object to generate a config object. When I mock this, instead of Foo I get:

<Mock name='mock().get_properties().property_c' id='1910891784064'>

Code:

# Real

class ClassC:
    property_c = "Foo"

class ClassB:
    def __init__(self):
        pass

    def login(self):
        print("Logged in...")

    def get_properties(self):
        return ClassC()

class ClassA:
    def __init__(self, conn):
        self.conn = conn
        self.conn.login()

a = ClassA(conn=ClassB()) >>> Logged in...
result = a.conn.get_properties()
print(result.property_c) >>> Foo


# Mocked

from unittest.mock import Mock
mock_b = Mock()
mock_c = Mock()
mock_c.property_c.return_value = "Foo_Mock"
mock_b.get_properties.return_value = mock_c

a = ClassA(conn=mock_b())
result = a.conn.get_properties()
print(result.property_c) >>> Output shown above

How do I mock this properly?

Edit1 - The suggested duplicate S.O answer only partially answers the question.

Edit2 - I forgot to include the mock login behaviour

mock_b.login.return_value = print("Logged in...") awesoon's answer still works with this modification:

    mock_b = Mock()
    mock_c = Mock()
    type(mock_c).property_c = PropertyMock(return_value="Foo_Mock")
    mock_b.get_properties.return_value = mock_c
    mock_b.login.return_value = print("Logged in...")

I see how PropertyMock can be used for ClassC, but how do I inject it into ClassB? — GlaceCelery
– GlaceCelery, Commented May 6, 2019 at 10:53
type(mock_c).property_c = PropertyMock(return_value="Foo_Mock") and mock_b.get_properties().property_c shall do the trick. Note I did not call mock_b() — awesoon
– awesoon, Commented May 6, 2019 at 10:59
I still get <Mock name='mock().get_properties().property_c' id='1910891784064'> — GlaceCelery
– GlaceCelery, Commented May 6, 2019 at 11:09

awesoon · Accepted Answer · 2019-05-07 05:23:01Z

2

There are two issues in your code:

Do not call mock_b when passing to ClassA (a = ClassA(conn=mock_b()). Calling a mock will create another mock and it will not contain changes made to mock_b.
Use PropertyMock to mimic attribute:
```
type(mock_c).property_c = PropertyMock(return_value="Foo_Mock")
```
You can also set attribute explicitly, but PropertyMock is more flexible.

The final code is:

from unittest.mock import Mock, PropertyMock
mock_b = Mock()
mock_c = Mock()
type(mock_c).property_c = PropertyMock(return_value="Foo_Mock")
mock_b.get_properties.return_value = mock_c

print(mock_b.get_properties().property_c)

Outputs:

Foo_Mock

answered May 7, 2019 at 5:23

awesoon

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Comments

Relandom · Accepted Answer · 2019-05-06 11:17:21Z

0

I think problem is with this line:

a = ClassA(conn=mock_b())

You are creating here new mock. If you try below code:

mock_b = Mock(name='mock_b')
a = ClassA(conn=mock_b())
print(mock_b)
print(a.conn)

You will see that those, are two different mocks, with different IDs (for example):

<Mock name='mock_b' id='48709360'>
<Mock name='mock_b()' id='48709528'>

So frits of all, you need to replace conn=mock_b() by conn=mock_b.

If you fix that then you could call:

print(result.property_c())  # notice extra parathesis.

It would look better if you add get_property method to ClassC, and then mock method get_property.

answered May 6, 2019 at 11:17

Relandom

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2 Comments

GlaceCelery Over a year ago

I cannot structure it your way. I'm bound by the api

Relandom Over a year ago

Ok, so you can simply change your 2 lines in your test: mock_c.property_c.return_value = "Foo_Mock" >> mock_c.property_c = "Foo_Mock" and: a = ClassA(conn=mock_b()) >> a = ClassA(conn=mock_b)

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Mock a nested object in Python using Mock()

Edit1 - The suggested duplicate S.O answer only partially answers the question.

Edit2 - I forgot to include the mock login behaviour

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Edit1 - The suggested duplicate S.O answer only partially answers the question.

Edit2 - I forgot to include the mock login behaviour

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