New submission from Ian Carr-de Avelon:
I'm new to Python and I've hit what appears to me to be a bug, but may be a
feature, so a tutorial bug.
I tried to loop through the items in a list, test each and remove those which
fail the test.
Simplifying to illustrate:
print test
[1, 2, 3, 4, 5]
Chris Jerdonek added the comment:
I have worked with languages where you are explicitly warned that you must
not mess with the loop variable
There is a warning in this part of the tutorial:
It is not safe to modify the sequence being iterated over in the loop...
(from
Chris Jerdonek added the comment:
Attached is a simple way of addressing this (essentially copying the verbiage
and example from the other page). If we want, we could make the sample code
different so that the reader doesn't see the same thing twice.
--
keywords: +patch
stage: -
Serhiy Storchaka added the comment:
It is safe to modify a sequence during iteration if it's size not increased.
words = ['cat', 'window', 'defenestrate']
for i, w in enumerate(words):
... if len(w) 6:
... words[i] = w[:5] + '…'
...
words
['cat', 'window', 'defen…']
--
Chris Jerdonek added the comment:
It is safe to modify a sequence during iteration if it's size not increased.
What do you mean by safe? The example given by the original commenter does
not increase the size either. I believe safe is meant in the sense of
avoiding possibly unexpected
Georg Brandl added the comment:
Well, I guess Serhiy meant neither increase nor decrease.
In the end, the exact behavior will never be clear if you don't state
explicitly how it's implemented: a counter that starts at 0 and is increased on
__next__ until it's equal to len(list) (as checked at
Chris Jerdonek added the comment:
But in general the advice should be: if you want to insert or remove elements
during iteration, iterate over a copy.
I would expand this to cover changing the list in any way. I think the point
being made is that iteration doesn't implicitly make a copy.
Chris Jerdonek added the comment:
Attaching revised patch.
--
Added file: http://bugs.python.org/file27573/issue-16225-2-default.patch
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http://bugs.python.org/issue16225
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Chris Jerdonek added the comment:
Reattaching. I duplicated a variable definition that was defined previously.
--
Added file: http://bugs.python.org/file27575/issue-16225-3-default.patch
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Serhiy Storchaka added the comment:
I mean it does not lead to crash, hang, etc. Even growing list during iteration
can be safe you move forward faster than list grows or if you known where to
stop.
Unexpected behavior for one people can be expected for others.
--
Chris Jerdonek added the comment:
I mean it does not lead to crash, hang, etc.
I agree. I removed the word safe in the patch I attached to reduce
ambiguity. Regarding unexpected behavior, remember that the tutorial is for
beginners/newcomers.
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Serhiy Storchaka added the comment:
I agree. I removed the word safe in the patch I attached to reduce
ambiguity.
Yes, so much the better.
It will be nice somewhere in deep clarify for experts what happens with list
iterator if the list changed.
And iterating over modifyed (if you
Chris Jerdonek added the comment:
It will be nice somewhere in deep clarify for experts what happens with list
iterator if the list changed.
There is a note somewhat to this effect here:
http://docs.python.org/dev/reference/compound_stmts.html#the-for-statement
Note There is a subtlety
Changes by Jesús Cea Avión j...@jcea.es:
--
nosy: +jcea
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