Difference between 'is' and '=='

[alex23]

Felipe Almeida Lessa wrote:
> I said [constants defined in your code] can (maybe "should"?) be used with "is", and
> AFAICT I'm right as well:
> >>> b = a
> >>> b is a
> True

You should _never_ use 'is' to check for equivalence of value. Yes, due
to the implementation of CPython the behaviour you quote above does
occur, but it doesn't mean quite what you seem to think it does.

Try this:

>>> UPPERLIMIT = 100
>>> i = 0
>>> while not (i is UPPERLIMIT):
>>>     i+=1
>>>     print i

Comparing a changing variable to a pre-defined constant seems a lot
more general a use case than sequential binding & comparison...and as
this should show, 'is' does _not_ catch these cases.

- alex23