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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Idiosyncratic python
Date: Thu, 24 Sep 2015 02:54:16 -0400
References: <560391ea$0$2885$c3e8da3$76491128@news.astraweb.com> <87twqkbavx.fsf@jester.gateway.sonic.net> <560399b0$0$1511$c3e8da3$5496439d@news.astraweb.com>
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Xref: csiph.com comp.lang.python:97064

On 9/24/2015 2:35 AM, Steven D'Aprano wrote:
> On Thursday 24 September 2015 16:16, Paul Rubin wrote:
>
>> Steven D'Aprano <steve+comp.lang.python@pearwood.info> writes:
>>> for k, v in mydict.items():
>>>      del(k)
>>
>> That looks wrong: it's deleting k from what?
>
> The local namespace.
>
> py> k = 23
> py> print k
> 23
> py> del k
> py> print k
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> NameError: name 'k' is not defined
>
>
>>> instead of the more obvious
>>> for v in mydict.values():
>>>      ...
>>
>> Maybe you mean
>>
>>     while mydict:
>>        k, v = mydict.popitem()
>>        ...
>
>
> Hah, no, you're the second person to make that mistake! One of the guys I
> work with suggested `mydict = {}` to empty the dict.
>
> `del k` (aside: no need for parens, del is a statement, not a function)
> doesn't delete the key from the dictionary. It just deletes the name k. The
> obvious intent is to iterate over the *values* of the dictionary, but the
> coder didn't know about values, so he iterated over (key,value) pairs, then
> deleted the key local variable (not the key in the dict!) to keep the
> namespace clean.

but, but, each iteration rebinds k, so del k is only needed after the 
loop .. unless one is super fanatic about cleanliness within the loop ;-)


-- 
Terry Jan Reedy