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Re: For-each behavior while modifying a collection

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On 29 Nov 2013 03:22:45 GMT, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> declaimed the following:


>Even if there was a counter, how should it be modified? The code you show 
>was this:
>
>def keepByValue(self, key=None, value=[]):
>    for row in self.flows:
>        if not row[key] in value:
>            self.flows.remove(row)
>
>
>What exactly does the remove() method do? How do you know?
>
>self.flows could be *any object at all*, it won't be known until run-
>time. The remove method could do *anything*, that won't be known until 
>runtime either. Just because you, the programmer, expects that self.flows 
>will be a list, and that remove() will remove at most one item, doesn't 
>mean that Python can possibly know that. Perhaps self.flows returns an 
>subclass of list, and remove() will remove all of the matching items, not 
>just one. Perhaps it is some other object, and rather than removing 
>anything, in fact it actually inserts extra items in the middle of the 
>sequence. (There is no law that says that methods must do what they say 
>they do.)
>

	Let's really confuse matters...

	Say "self.flows" is something derived from a database cursor/result
set...

	Then "self.flows.remove(row)" could: 1) remove the row from the
cursor/result set [iterating over a cursor tends, in my experience, to use
up the items]; 2) execute a query to remove the matching row from the
database itself; 3) do both...
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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Thread

For-each behavior while modifying a collection Valentin Zahnd <v.zahnd@gmail.com> - 2013-11-28 16:49 +0100
  Re: For-each behavior while modifying a collection Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-11-29 03:22 +0000
    Re: For-each behavior while modifying a collection Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-11-29 11:37 -0500

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