pytest spy on inner class method

Daniel Parker

3 min read

·

24-02-2025

1

0

Share

Testing inner class methods can be tricky. Standard mocking techniques might not always work seamlessly. This article explores effective strategies using pytest to spy on and verify interactions with methods within inner classes, ensuring thorough test coverage. We'll cover several approaches, each with its strengths and weaknesses.

Understanding the Challenge

Inner classes, while useful for encapsulation, present a unique challenge for testing. Directly mocking the inner class method might require complex mocking setups. Instead, spying offers a cleaner, more focused approach. Spying involves observing method calls without altering their behavior. This allows us to verify that a method was called, how many times, and with what arguments, without interfering with the inner class's internal logic.

Method 1: Using pytest-mock for Direct Spying

The pytest-mock fixture provides a powerful way to directly mock and spy on inner class methods.

import pytest

class OuterClass:
    class InnerClass:
        def inner_method(self, arg1, arg2):
            return arg1 + arg2

    def __init__(self):
        self.inner = self.InnerClass()

def test_outer_class_with_spy(mocker):
    outer = OuterClass()
    spy = mocker.spy(outer.inner, 'inner_method')  # Spy on the inner method

    result = outer.inner.inner_method(5, 3)

    assert result == 8
    spy.assert_called_once_with(5, 3) #Verify call with specific arguments
    spy.assert_called_once() #Verify it was called once

This example directly spies on inner_method using mocker.spy. We then use assertion methods from the spy object to verify the call count and arguments. This approach is clean and intuitive.

Method 2: Mocking the Inner Class Instance

If you don't want to directly spy on the method but rather control the inner class's behavior, mocking the inner class instance offers more control.

import pytest

class OuterClass:
    class InnerClass:
        def inner_method(self, arg1, arg2):
            return arg1 + arg2

    def __init__(self):
        self.inner = self.InnerClass()

def test_outer_class_mock_inner(mocker):
    mock_inner = mocker.Mock()
    mock_inner.inner_method.return_value = 10 # Set a return value for the mock

    outer = OuterClass()
    outer.inner = mock_inner # Replace the real inner class with the mock

    result = outer.inner.inner_method(5,3)
    assert result == 10
    mock_inner.inner_method.assert_called_once()

This approach replaces the actual inner class instance with a mock. This gives more control over the inner method's behavior. This method is suitable when you need to control the return value or raise exceptions from the inner method during testing.

Method 3: Indirect Spying Through the Outer Class

For scenarios where directly accessing the inner class is challenging, spying can be achieved indirectly through the outer class's methods.

import pytest

class OuterClass:
    class InnerClass:
        def inner_method(self, arg1, arg2):
            return arg1 + arg2

    def __init__(self):
        self.inner = self.InnerClass()

    def outer_method(self, arg1, arg2):
        return self.inner.inner_method(arg1, arg2)

def test_outer_method_indirect_spy(mocker):
    outer = OuterClass()
    spy = mocker.spy(outer, 'outer_method')

    result = outer.outer_method(5,3)

    assert result == 8
    spy.assert_called_once_with(5, 3)

Here, we spy on the outer class method that indirectly calls the inner class method. This is useful when the inner class is not directly accessible or when the testing focus is on the overall interaction.

Choosing the Right Approach

The best approach depends on your specific testing needs.

• Direct Spying (pytest-mock): Ideal when you need to precisely verify the inner method's calls and arguments without altering its behavior.

• Mocking the Inner Class: Suitable when you need to control the inner method's return value or simulate exceptions.

• Indirect Spying: Useful when direct access to the inner class is limited or when the testing focus is on the higher-level interaction.

Remember to install pytest and pytest-mock: pip install pytest pytest-mock

By carefully selecting the right approach, you can effectively use pytest to comprehensively test interactions with inner class methods, ensuring robust and reliable software.