How does assertGreater and assertLess know an instance of an object is greater or lesser than the other

Question

import unittest

import moves


class MoveTests(unittest.TestCase):

    def setUp(self):
      self.rock = moves.Rock()
      self.paper= moves.Paper()
      self.scrissors = moves.Scissors()

    def test_rock_better_than_scissors(self):
      self.assertGreater(self.rock, self.scrissors)

    def test_paper_worse_than_scissors(
      self):
      self.assertLess(self.paper, self.scrissors)

Answer 1 · 2015-05-06T19:41:20Z

May 6, 2015 7:41pm

Hi Welby,

I believe it's the __gt__ and __lt__ methods that allow the comparison to happen. You have to supply these in your class so that 'greater than' and 'less than' comparisons can be made between objects.

In general:

assertGreater(x, y)

#Checks if
x > y

# This calls
x.__gt__(y)

In the case of the code you posted:

self.assertGreater(self.rock, self.scrissors)

The __gt__ method would be called on the rock object and the scissors object would be passed in as the argument.

This is the moves.py file from the "rps" folder related to this course:

moves.py

class Move:
    better_than = None
    worse_than = None

    def __gt__(self, other):
        """Is this instance being compared to an instance from a worse class"""
        return other.__class__.__name__ in self.better_than

    def __lt__(self, other):
        """Is this instance being compared to an instance from a better class"""
        return other.__class__.__name__ in self.worse_than

    def __eq__(self, other):
        """Is this instance being compared to an instance from the same class"""
        return type(other) == type(self)

    def __ne__(self, other):
        """Is this instance being compared to an instance from another class"""
        return other.__class__ != self.__class__


class Rock(Move):
    better_than = ['Scissors']
    worse_than = ['Paper']


class Paper(Move):
    better_than = ['Rock']
    worse_than = ['Scissors']


class Scissors(Move):
    better_than = ['Paper']
    worse_than = ['Rock']

Kenneth did supply both a __gt__ and __lt__ method which will return True or False depending on the result of the return statement.

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Welby Obeng

Welby Obeng

How does assertGreater and assertLess know an instance of an object is greater or lesser than the other

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Jason Anello

Jason Anello