Monkeypatching class method in python2

Question

I have module named square.py:

import math
class Square(object):
 def __init__(radius):
 self.radius = radius
 def calculate_area(self):
 return math.sqrt(self.radius) * math.pi 

And I have written test for this using py.test:

from square import Square
def test_mocking_class_methods(monkeypatch):
 monkeypatch.setattr('test_class_pytest.Square.calculate_area', lambda: 1)
 assert Square.calculate_area() == 1

Running this test in python 2 gives me the following output:

> assert Square.calculate_area() == 1
E TypeError: unbound method <lambda>() must be called with Square instance as first argument (got nothing instead)

But the same test in python 3 passes. Do you know why is that and how can I fix this test to work with python 2?

Answer 1

You need to call calculate_area() on an instane, but you called it on the Square class. You never created a square to calculate the area of.

Answer 2

Python 2 ensures that a method on a class is always called with the first argument being an instance of that class (Usually called self).

So, it is expecting something like this:

Square().calculate_area()
# Whih is equivalent to
Square.calculate_area(Square())

But this will also throw an error, as it is an unexpected argument (TypeError: <lambda>() takes no arguments (1 given))

To prevent checking for the self parameter, use the staticmethod decorator:

monkeypatch.setattr('test_class_pytest.Square.calculate_area', staticmethod(lambda: 1))