In what situation is an object not equal to itself?

Question

I found the following lines in the json/encoder.py module:

if o != o:
   text = 'NaN'

This presumably checks if `o` is a `NaN`. Since a `NaN` satisfies the above condition — yatu, Dec 09 '19 at 17:06
You can define the behavior of any type using the `__eq__` hook — juanpa.arrivillaga, Dec 09 '19 at 17:29

score 7 · Accepted Answer · answered Dec 09 '19 at 17:08

7

This can happen in the case of floating point numbers that adhere to IEEE 754 standard. See Why is NaN not equal to NaN?

By definition, a value of NaN ("not a number") is unequal to itself.

answered Dec 09 '19 at 17:08

lukeg

score 5 · Answer 2 · answered Dec 09 '19 at 17:42

The question seems to be about NaN, but it's worth mentioning that you can define the comparison method __eq__ in a custom class.

For example you could make it always false:

class NotEqual:
    def __eq__(self, other):
        return False

n = NotEqual()
print(n == n)  # -> False

2 Answers2