Comparison with False - something I don't understand

[Nobody]

On Thu, 02 Dec 2010 07:28:30 +0000, Harishankar wrote:

> When I run pychecker through my modules I get the message that 
> comparisons with "False" is not necessary and that it might yield 
> unexpected results.
> 
> Yet in some situations I need to specifically check whether False was 
> returned or None was returned. Why is comparison with False so bad?

The behaviour may be counterintuitive.

One might expect that "x == False" is equivalent to "not x". Sometimes it
is, sometimes it isn't.

E.g. 0 and 0.0 are equal to False and are equivalent to False when
converted to booleans:

	> 0 == False
	True
	> not 0
	True

	> 0.0 == False
	True
	> not 0.0
	True

[], "" and None aren't equal to False but are equivalent to False when
converted to booleans:

	> [] == False
	False
	> not []
	True

	> "" == False
	False
	> not ""
	True

	> None == False
	False
	> not None
	True

The boolean conversions are what's relevant for "if x ...", "while x ...",
etc.

If you want to test specifically for True, False or None, use "is" rather
than an equality check. This eliminates the warning and doesn't
risk misleading someone reading the code.