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if <assignment>:

Bengt Richter bokr at oz.net
Sun Nov 24 17:25:00 EST 2002 
On Sun, 24 Nov 2002 13:52:12 -0800, Erik Max Francis <max at alcyone.com> wrote:

>André Næss wrote:
>
>> if myvar = someFunction():
>> 
>> My question is, what is the rationale for this? Is it a technical
>> issue? Or
>> purely a matter of language design? I'm curious because I'm interested
>> in
>> the design og programming languages, not because I want this behavior
>> changed in Pyton :)
>
>It's a language design feature, intended to avoid the confusion between
>when people write (say, in C):
>
>	if (a = b) ...
>
>but mean
>
>	if (a == b) ...
>
>Instead in Python assignments are treated as statements, not
>expressions, so they cannot appear in an if clause.
>
except in perverse ways (and this is not the only one):

 >>> a
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
 NameError: name 'a' is not defined
 >>> b=123
 >>> if [a for a in [b]][0] == 123: print 'aha'
 ...
 aha
 >>> a
 123

BTW, are list comprehensions going to get their own local namespace sometime
or is this the way it's going to be forever?

Regards,
Bengt Richter
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