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?

Amit Gold · Accepted Answer · 2016-04-23 20:09:17Z

3

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.

answered Apr 23, 2016 at 20:09

Amit Gold

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Artyer · Accepted Answer · 2017-08-06 11:02:26Z

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))

answered Aug 6, 2017 at 11:02

Artyer

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