Python Mocking with unitest.mock 2

Internal

• Python Mocking with unitest.mock

Mocking a Method

The general approach is to replace at runtime the real method instance associated with the class instance to be tested with a Mock instance, configured to simulate various behaviors of the real method.

Assuming that our dependency to test with is SomeClass, and this class has a some_method(nuance: str) whose behavior we want to mock during testing, the initial implementation of the class and method could be:

 
class SomeClass:
    def __init__(self, color: str):
        self._color = color

    def some_method(self, nuance: str) -> str:
        return f'{nuance} {self._color}'.upper()

The normal behavior of the method some_method(nuance: str) is reflected by:

 
c = SomeClass('blue')
assert c.some_method('dark') == 'DARK BLUE'

c._color = 'red'
assert c.some_method('light') == 'LIGHT RED'

We can mock the behavior of the some_method(nuance: str) method in the following ways:

• We can return a constant value, regardless of the arguments the method is invoked with.
• We can raise an exception, regardless of the arguments the method is invoked with.
• We can replace the behavior of the method with custom logic, that takes into account the arguments the method is called with.

Simulating a Particular Return Value Irrespective of the Arguments it was Called With

Configure Mock() using a return_value argument so no matter how the mocked method is invoked, it will always return a constant value:

c = SomeClass('blue')
c.some_method = Mock(return_value='something completely arbitrary')
assert c.some_method('argument does not matter') == 'something completely arbitrary'

Irrespective of how the method is invoked in testing, the calling code will always get the value configured with return_value on the mock.