Is Python’s == an equivalence relation on the floats?

== is reflexive for all numbers, zero, -zero, ininity, and -infinity, but not for nan.

You can get inf, -inf, and nan in native Python just by arithmetic operations on literals, like below.

These behave correctly, as in IEEE 754 and without math domain exception:

>>> 1e1000 == 1e1000
True
>>> 1e1000/1e1000 == 1e1000/1e1000
False

1e1000 is a very big number, so float and double represent it as an infinity.

• infinity is equal to infinity
• infinity divided by infinity is not a number
• not a number != not a number

Floating-point arithmetic in Python also works OK for infinity minus infinity etc.:

>>> x = 1e1000
>>> x
inf
>>> x+x
inf
>>> x-x
nan
>>> x*2
inf
>>> x == x
True
>>> x-x == x-x
False
>>> 

And for the zero and minus zero case:

>>> inf = float("inf")
>>> 1/inf
0.0
>>> -1/inf
-0.0
>>> -1/inf == 1/inf
True
>>>