Message from discussion 0 == False but [] != False?

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From: Rex Turnbull <rex@no_spam.dicad.de>
Newsgroups: comp.lang.python
Subject: Re: 0 == False but [] != False?
Date: Thu, 24 May 2007 13:21:50 +0200
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Steven D'Aprano :
> On Thu, 24 May 2007 06:59:32 +0000, Tim Roberts wrote:
> 
>> As a general rule, I've found code like "if x == False" to be a bad idea in
>> ANY language.
> 
> 
> Surely that should be written as "if (x == False) == True"?
> 
> 
Why compare to False?

" if not x : ... "

It really doesn't matter if x is False or if it evaluates to False. Many 
things evaluate to False like [], (), 0, "", None and a few other things.



 >>> def tf(thing):
...     if thing : print "True thing", thing
...     elif not thing : print "False thing",thing
...     else : print "No thing"
...
 >>> tf([])
False thing []
 >>> tf([1])
True thing [1]
 >>> a = ()
 >>> tf(a)
False thing ()
 >>> a=(0)
 >>> tf(a)
False thing 0
 >>> a= (1,2,3)
 >>> tf(a)
True thing (1, 2, 3)
 >>> tf("abc")
True thing abc
 >>> tf("")
False thing
 >>>