[Python-ideas] checking for identity before comparing built-in objects

[Max Moroz]

On Thu, Oct 4, 2012 at 7:19 AM, MRAB <python at mrabarnett.plus.com> wrote:
> Think of it this way:
>
> Calculation A returns NaN for some reason
>
> Calculation B also returns NaN for some reason
>
> Have they really returned the same result? Just because they're both
> NaN doesn't mean that they're the _same_ NaN...

Someone who performs two calculations with float numbers should never
compare their results for equality. It's really a bug to rely on that
comparison:

# this is a bug
# since the result of this comparison for regular numbers is unpredictable
# so doesn't it really matter how this behaves when NaNs are compared?
if a/b == c/d:
    # ...

On the other hand, comparing a number to another number, when none of
the two numbers are involved in a calculation, is perfectly fine:

# this is not a bug
# too bad that it won't work as expected
# when input1 == input2 == 'nan'
a = float(input1)
b = float(input2)
if a == b:
    # ...

So it seems to me your argument is this: "let's break the expectations
of developers who are writing valid code, in order to partially meet
the expectations of developers who are writing buggy code". If so, I disagree.