Question

A person asked a question on SO about how to get a list unique function in python with an alternative equality function.

I was thinking it could be done by inheriting from the element class and overloading the equality function

import functools
@functools.total_ordering
class ffloat(float):
def __eq__(self,other):
    if floor(self) == floor(other):
        return True
    else:
        return False
def __le__(self,other):
    if self == other:
        return True
    else:
        return float(self) <= float(other)
def __hash__(self):
    return floor(self)


a = map(ffloat,[4.3,8,8.9, 13])


In [41]: a[1] == a[2]
Out[41]: True

but

In [42]: set(a)
Out[42]: set([4.3, 8.0, 8.9, 13.0])

Edit: replaced abs < 1.5 equality with floor equality

Added Hash
P.S. is there a way to make a class factory out of this that a class and two lambda and returns a class that inherits from the first one overriding the needed equality function.

Answer 1

This is not a valid equality function, since it’s not transitive:

mfloat(0) == mfloat(1) == mfloat(2), but mfloat(0) != mfloat(2).

Also note that in order to be used in a set, you must override __hash__ so that the following property holds for all instances a, b of your class:

a == b ⇒ hash(a) == hash(b)

set finds out that hash(mfloat(8)) != hash(mfloat(9)). Since set assumes the above property holds, it concludes that mfloat(8) != mfloat(9) without actually calling __eq__.

---

In summary, this works:

from math import floor
class ffloat(float):
    def __eq__(self,other):
        return floor(self) == floor(other):
    def __hash__(self):
        return floor(self)

a = map(ffloat,[4.3,8,8.9, 13])
print(set(a))
# output: {8.0, 4.3, 13.0}