pytest spy on method

Charlotte

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26-02-2025

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Testing interactions between objects is crucial for robust software. When a method's internal logic is complex or involves external dependencies, directly testing it can be challenging and brittle. This is where pytest's mocking capabilities, specifically spying on methods, become invaluable. This guide will explore how to effectively spy on methods using pytest, improving the clarity and maintainability of your tests.

Why Spy on Methods?

Spying, a form of mocking, allows you to observe how a method is called without altering its behavior. This differs from patching or stubbing, which replace the method's functionality entirely. Spying provides valuable information on:

• Call Count: How many times a method was called.
• Arguments: The values passed to the method during each call.
• Return Value: The value the method returned (if any).

This information is crucial for verifying interactions, identifying unexpected behavior, and ensuring your code works as intended.

Using pytest-mock for Method Spying

The pytest-mock plugin provides a straightforward way to spy on methods. Let's explore this with an example. Assume you have a class MyClass:

class MyClass:
    def method_to_spy_on(self, arg1, arg2):
        result = arg1 + arg2
        return result

    def another_method(self):
        return self.method_to_spy_on(5, 10)

We want to test another_method, but we're primarily interested in verifying that method_to_spy_on is called correctly.

import pytest
from unittest.mock import call

def test_another_method(mocker):
    spy = mocker.spy(MyClass, 'method_to_spy_on')
    my_instance = MyClass()
    my_instance.another_method()

    assert spy.call_count == 1
    spy.assert_called_once_with(5, 10)  # Verifies arguments passed
    assert my_instance.another_method() == 15  # Verify return value from another_method

In this test:

1. mocker.spy(MyClass, 'method_to_spy_on') creates a spy on the specified method.
2. spy.call_count checks how many times the method was called.
3. spy.assert_called_once_with(5, 10) asserts that the method was called exactly once with the arguments 5 and 10. Other assertion methods like assert_called_with and assert_any_call are available for more complex scenarios.

Handling Different Return Values

Sometimes you might need to control the return value of the spied method during testing without changing its actual implementation. This can be achieved using side_effect:

def test_another_method_with_side_effect(mocker):
    spy = mocker.spy(MyClass, 'method_to_spy_on')
    spy.side_effect = lambda x, y: x * y  # Custom logic for the spy
    my_instance = MyClass()
    result = my_instance.another_method()

    assert spy.call_count == 1
    assert result == 50 # result of 5 * 10 from side_effect

Here, side_effect redirects calls to method_to_spy_on through a lambda function, allowing us to control the returned value.

Spying on Methods within a Class Instance

You can also spy on methods belonging to a specific instance of a class:

def test_spy_on_instance_method(mocker):
    my_instance = MyClass()
    spy = mocker.spy(my_instance, 'method_to_spy_on')
    my_instance.method_to_spy_on(2,3)
    assert spy.call_count == 1
    assert spy.call_args == call(2,3)

This approach is useful when dealing with methods that rely on the instance's internal state.

Beyond Basic Spying: Advanced Techniques

pytest-mock offers many more advanced techniques for mocking and spying. Explore the documentation to learn about:

• Patching: Replacing a method with a mock object.
• Stubbing: Defining a specific return value for a method call.
• Magic Methods: Spying on special methods like __init__.

Conclusion

Method spying with pytest-mock is a powerful tool for writing more focused and reliable tests. It allows you to verify interactions between objects without needing to mock their entire implementation. By combining spying with other mocking techniques, you can build comprehensive test suites that uncover subtle bugs and ensure the quality of your code. Remember to install pytest-mock using pip install pytest-mock.