pytest spy on inner class method return

Charlotte

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01-03-2025

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Testing inner class methods can be tricky. This article shows you how to effectively spy on the return values of inner class methods using pytest's monkeypatch fixture. We'll cover different scenarios and best practices for robust testing.

Understanding the Challenge

Let's say we have a class with an inner class containing a method whose behavior we want to control during testing. Directly mocking the inner class method is often cumbersome and can lead to brittle tests. Instead, we leverage pytest's monkeypatch to achieve clean and effective spying.

Example Scenario: A Class with an Inner Class

Consider this example:

class OuterClass:
    class InnerClass:
        def some_method(self, arg):
            # Some complex logic here
            return arg * 2

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

    def call_inner_method(self, arg):
        return self.inner.some_method(arg)

We want to test OuterClass.call_inner_method, but we're only interested in verifying that InnerClass.some_method is called with the correct argument and that the result is handled appropriately. We don't want to test the internal workings of InnerClass.some_method itself.

Using monkeypatch to Spy

Here's how we can use monkeypatch to spy on the return value of InnerClass.some_method:

import pytest

class OuterClass:
    class InnerClass:
        def some_method(self, arg):
            return arg * 2

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

    def call_inner_method(self, arg):
        return self.inner.some_method(arg)

def test_outer_class_method(monkeypatch):
    # Create a spy function to track the call and return a controlled value
    spy_method_call_count = 0
    def spy_some_method(arg):
        nonlocal spy_method_call_count
        spy_method_call_count += 1
        return 10  # Controlled return value

    # Apply the spy to the inner class method
    monkeypatch.setattr(OuterClass.InnerClass, 'some_method', spy_some_method)

    outer = OuterClass()
    result = outer.call_inner_method(5)

    # Assertions
    assert result == 10
    assert spy_method_call_count == 1

This test does the following:

1. Creates a spy function: spy_some_method tracks calls and returns a controlled value (10 in this case).
2. Applies the spy: monkeypatch.setattr replaces the original some_method with our spy.
3. Calls the outer class method: The test calls call_inner_method, indirectly exercising the spy.
4. Assertions: The test verifies both the return value from call_inner_method and the number of times spy_some_method was called.

Handling Different Return Types

The spy function can return any data type. For example:

def spy_some_method(arg):
    return {"result": arg * 2}

More Complex Scenarios

For more intricate scenarios, where you might need to control the behavior based on input arguments, you can add logic inside your spy function. For example:

def spy_some_method(arg):
    if arg > 5:
        return "Large Value"
    else:
        return "Small Value"

Best Practices

• Keep spies focused: Each spy should test a specific aspect of the inner class method's behavior. Avoid overly complex spies.
• Clear naming: Use descriptive names for your spy functions to indicate their purpose.
• Isolate tests: Ensure each test uses a separate monkeypatch fixture to avoid unintended side effects.

This approach provides a clean and effective way to test interactions between outer and inner classes without overcomplicating your tests. Using monkeypatch allows for focused testing on specific behaviors, improving maintainability and readability of your test suite. Remember to adapt these techniques to your specific class structure and testing requirements.