Patching Class Methods in Python: Accessing the cls Variable
Problem: You need to modify the behavior of a class method in Python, but you also need to access the cls argument (which represents the class itself) within the patched method.
Rephrased: You want to change how a class method works, but you also need to use the class itself inside the modified function.
Scenario: Imagine you have a class MyClass with a class method greet:
class MyClass:
def __init__(self, name):
self.name = name
@classmethod
def greet(cls, message):
print(f"{message} from {cls.__name__}!")
You want to patch the greet method to add a custom prefix to the message. However, you also need to access the class name (cls.__name__) within the patched function.
Solution: Use the patch function from the unittest.mock module. Here's how:
from unittest.mock import patch
class MyClass:
# ... (same as above)
@patch('__main__.MyClass.greet', autospec=True)
def test_greet(mock_greet):
prefix = "Custom "
mock_greet.side_effect = lambda cls, message: print(f"{prefix}{message} from {cls.__name__}!")
MyClass.greet("Hello") # Output: Custom Hello from MyClass!
Explanation:
- Import
patch: We import thepatchfunction fromunittest.mock. - Decorate the test function: We use
@patchas a decorator, specifying the path to the method (__main__.MyClass.greet) and enablingautospecfor correct type checking. - Create a side effect: We define a side effect for the mocked method. The side effect receives the
clsargument (the class itself) and themessageargument, allowing us to access the class name (cls.__name__) and modify the message with the custom prefix. - Call the original method: Finally, we call the original
greetmethod, which now uses the patched implementation with the custom prefix.
Key Points:
unittest.mockis powerful for testing, but can also be used to patch existing code.autospechelps ensure the mocked method behaves like the original.- You can use the
side_effectattribute of the mocked method to customize its behavior.
Additional Value:
- Alternative: If you need to patch a method that doesn't have a
clsargument, you can use thepatch.objectmethod:
from unittest.mock import patch
@patch.object(MyClass, 'greet', autospec=True)
def test_greet(mock_greet):
# ... (rest of the code is the same)
- Real-world use case: This technique is useful when you need to temporarily change the behavior of a class method, particularly when you need access to the class itself within the patched method. This could be useful for testing, debugging, or even for implementing dynamic behavior.
References:
