For-each behavior while modifying a collection

MRAB python at
Thu Nov 28 18:25:05 CET 2013

On 28/11/2013 17:20, Ned Batchelder wrote:
> On 11/28/13 10:49 AM, Valentin Zahnd wrote:
>> Hello
>> For-each does not iterate ober all entries of collection, if one
>> removes elements during the iteration.
>> Example code:
>> def keepByValue(self, key=None, value=[]):
>>      for row in self.flows:
>>          if not row[key] in value:
>>              self.flows.remove(row)
>> It is clear why it behaves on that way. Every time one removes an
>> element, the length of the colleciton decreases by one while the
>> counter of the for each statement is not.
>> The questions are:
>> 1. Why does the interprete not uses a copy of the collection to
>> iterate over it? Are there performance reasons?
> Because implicit copying would be pointless in most cases.  Most loops
> don't even want to modify the collection, why copy all iterables just in
> case your loop might be one of the tiny few that might change the
> collection?
> Of course, if that prices is acceptable to you, you could do the copy
> yourself:
>       for row in list(self.flows):
>           if row[key] not in value:
>               self.flows.remove(row)
>> 2. Why is the counter for the iteration not modified?
> Because the list and the iterator over the list are different objects.
> I suppose the list and the iterator could have been written to update
> when the list is modified, but it could get pretty complicated, even
> more so if you want to do the same for other collections like dictionaries.
> The best advice is: don't modify the list, instead make a new list:
>       self.flows = [r for r in self.flows if r[key] not in value]
> Be careful though, since there might be other references to the list,
> and now you have two.
The simple solution in that case is:

     self.flows[:] = [r for r in self.flows if r[key] not in value]

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