eq method returns True for both == and > operators

Question

I have declared a class Triangle in Python which takes base and height as __init__ arguments and has a method area that computes and returns the area of the triangle. The __eq__ method of the Triangle class compares the area of the triangles and return the value. The class is defined as below:

class Shape(object):
    def area(self):
        raise AttributeException("Subclasses should override this method.")

class Triangle(Shape):
    def __init__(self, base, height):
        """
        base = base of the triangle.
        height = height of the triangle
        """
        self.base = base
        self.height = height
    def area(self):
        return (self.base * self.height)/2
    def __str__(self):
        return 'Triangle with base ' + str(self.base) + ' and height ' + str(self.height)
    def __eq__(self, other):
        """
        Two triangles are equal if their area is equal
        """
        return (self.area() == other.area())

Now I ran the program and created two instances to Triangle t1 and t3, and gave them different base and height but their area is equal. So t1 == t3 should be True which is returned as True only. But strangely, t1 > t3 is also returned as True, which I don't understand why?

>>> t1 = Triangle(3,4)
>>> t3 = Triangle(2, 6)
>>> t1.area()
6
>>> t3.area()
6
>>> t1 == t3
True
>>> t1 > t3
True
>>> t1 < t3
False
>>>

You haven't shown enough of your class to reproduce the problem. — L3viathan, Mar 11 '17 at 16:02
Added the whole class. Its a simple learning example, I just defined __eq__. — Gaurav Sachdeva, Mar 11 '17 at 16:04
No Shape is a simple class inheriting Object and area method which does nothing but ask the child class to define area method. Added Shape as well. — Gaurav Sachdeva, Mar 11 '17 at 16:06
I still can't reproduce the problem with the code you posted. I get a `TypeError: '>' not supported between instances of 'Triangle' and 'Triangle'`. — L3viathan, Mar 11 '17 at 16:08
I don't get any error at all. I am using Python version 2.7.13. Are you using some other version? — Gaurav Sachdeva, Mar 11 '17 at 16:09
Ah, that is the problem. I can reproduce it with Python 2.7, just not with Python 3. Investigating. — L3viathan, Mar 11 '17 at 16:09

L3viathan · Accepted Answer · 2017-03-11T17:09:46.907

3

Although I can't find a source on this, it seems as if Python 2 uses the id to compare two objects if you don't define the respective magic methods yourself.

Observe:

>>> t1 = Triangle(3, 4)
>>> t3 = Triangle(2, 6)
>>> t1 > t3
False
>>> t1 < t3
True
>>> t1 = Triangle(3, 4)
>>> t1 > t3
True
>>> t1 < t3
False

This behaviour isn't guaranteed as id doesn't promise you to assign increasing numbers to objects created later, but it does usually seem to work in CPython 2.7, since id will give you the physical address of the object.

You will observe that before the redefinition of t1, id(t1) < id(t3), while afterwards, the opposite is the case.

edited Mar 11 '17 at 17:09

answered Mar 11 '17 at 16:13

L3viathan

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You mean that in CPython, it is guaranteed that new objects are given increasing number, so it will be consistent in Cpython but not guaranteed in Python? – Gaurav Sachdeva Mar 11 '17 at 16:45
2

@GauravSachdeva No such thing is guaranteed. In CPython, `id` is the physical address of the object. It can be any address at which there is space available (usually, that gives increasing numbers in the beginning...) – zvone Mar 11 '17 at 17:02

score 2 · Answer 2 · edited May 23 '17 at 12:25

According to the docs

If no __cmp__(), __eq__() or __ne__() operation is defined, class instances are compared by object identity (“address”).

Also stated in this answer, the newer versions of python 2 default uses the object's id for comparison.

So you should always define either __gt__ or __lt__ alongside of __eq__ for comparing the way you wanted. And if you want, you can also define __le__, __ge__, __ne__...

One simple thing you can change is:

class Triangle(Shape):
    def __init__(self, base, height):
        """
        base = base of the triangle.
        height = height of the triangle
        """
        self.base = base
        self.height = height
    def area(self):
        return (self.base * self.height)/2
    def __str__(self):
        return 'Triangle with base ' + str(self.base) + ' and height ' + str(self.height)
    def __eq__(self, other):
        """
        Two triangles are equal if their area is equal
        """
        return (self.area() == other.area())
    def __gt__(self, other):
        """
        This works for both greater than and less than
        """
        return (self.area() > other.area())

You probably want to define all of `__gt__`, `__lt__`, `__ge__`, `__le__`, `__eq__`, `__ne__` at the same time, or use [total_ordering](https://docs.python.org/2/library/functools.html#functools.total_ordering). More importantly, you should always implement `__hash__` when you implement `__eq__`, or things (ike `dict`) will break unexpectedly. — zvone, Mar 11 '17 at 17:09

__eq__ method returns True for both == and > operators

2 Answers2

eq method returns True for both == and > operators