[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Stefan Behnel  added the comment:

Closing since this works as designed.

Users are responsible for avoiding concurrent tree modifications during 
iteration.

--
resolution:  -> not a bug
stage: patch review -> resolved
status: open -> closed

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Ned Deily]

Ned Deily  added the comment:


New changeset d5719247ba32b3ac700454bad9a9e2cc7edeac6a by Miss Skeleton (bot) 
in branch '3.9':
bpo-41892: Clarify that an example in the ElementTree docs explicitly avoids 
modifying an XML tree while iterating over it. (GH-22464) (GH-22554)
https://github.com/python/cpython/commit/d5719247ba32b3ac700454bad9a9e2cc7edeac6a


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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[miss-islington]

miss-islington  added the comment:


New changeset 6bd058e0ff5d4a63fb35f6d45161cdf51cb68c58 by Miss Skeleton (bot) 
in branch '3.8':
bpo-41892: Clarify that an example in the ElementTree docs explicitly avoids 
modifying an XML tree while iterating over it. (GH-22464)
https://github.com/python/cpython/commit/6bd058e0ff5d4a63fb35f6d45161cdf51cb68c58


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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[miss-islington]

Change by miss-islington :


--
pull_requests: +21552
pull_request: https://github.com/python/cpython/pull/22554

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 7.0 -> 8.0
pull_requests: +21551
pull_request: https://github.com/python/cpython/pull/22553

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Ned Deily]

Ned Deily  added the comment:


New changeset 40db798692ca783fc2163656f196ac77e8b9e792 by scoder in branch 
'master':
bpo-41892: Clarify that an example in the ElementTree docs explicitly avoids 
modifying an XML tree while iterating over it. (GH-22464)
https://github.com/python/cpython/commit/40db798692ca783fc2163656f196ac77e8b9e792


--
nosy: +ned.deily

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Change by Stefan Behnel :


--
pull_requests: +21547
pull_request: https://github.com/python/cpython/pull/22546

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Change by Stefan Behnel :


--
versions: +Python 3.10, Python 3.9

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Eric V. Smith]

Eric V. Smith  added the comment:

I can't say how ElementTree works without more checking, but this solution 
cannot work in general. Given a pointer to an object that's in a list, how 
would you get to the next item? Say the parent list-like object has a C array 
of pointers to the objects it contains, and removing one of the objects 
re-shuffles that list. How would keeping a pointer to the current object help 
you?

In any event, I don't think this behavior is going to change.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

WoodyWoo  added the comment:

Could I say the mutable sequence containing not the object but the pointer like 
C++.
So they can changed in def functions.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

WoodyWoo  added the comment:

@eric.smith @scoder @serhiy.storchaka Thank U all.
I get what to do,and still think the "for in" structure should rebuilding.
All three methods:

import xml.etree.ElementTree as ET 
xmlstr=\
r'''


2
2008
141100




5
2011
59900



69
2011
13600




69
2011
13600



'''
print(xmlstr)

#orginal code
root = ET.fromstring(xmlstr)
for country in root.findall('country'):
rank = int(country.find('rank').text)
if rank > 50:
root.remove(country)
print("___orginal___")
for country in root.findall('country'):
print (country.get("name"))
print("^^^orginal\n")

#wrong code in my mind
root = ET.fromstring(xmlstr)
for country in root:
rank = int(country.find('rank').text)
if rank > 50:
root.remove(country)
print("___bad___")
for country in root.findall('country'):
print (country.get("name"))
print("^^^bad\n")

#my code
root = ET.fromstring(xmlstr)
index=0
count=len(root.findall("./*"))
while index 50:
root.remove(root[index])
index=index+0
count=count-1  # avoid index err
continue
index=index+1
print("___new___")
for country in root.findall('country'):
print (country.get("name"))
print("^^^new\n")

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

WoodyWoo  added the comment:

I think "for each in list" should not work by index,but should work by pointer 
like thing,could be a list of pointers.
When the "for each in list" was acting,the pointers list would be fixed,and you 
need not to worry about the "list" changing.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Stefan Behnel  added the comment:

@WoodyWoo, you can (and should) do something like this:

for ELEMchild in list(etroot):

Modifying a container while iterating over it is usually not a good idea. The 
same applies to Python lists and dicts, for example.

--
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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Change by Stefan Behnel :


--
stage:  -> patch review

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Change by Stefan Behnel :


--
keywords: +patch
pull_requests: +21489
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/22464

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Eric V. Smith]

Eric V. Smith  added the comment:

The example is iterating over the list returned by root.findall(), but removing 
from a different data structure in root, so it won't have a problem.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

WoodyWoo  added the comment:

Only that makes "for each in list" literally.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

WoodyWoo  added the comment:

I'm green hand in Coding.
But I think the problem caused by "for each in list".
Anything changed in "list" should not act at once,but should act when "for end" 
or break.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Eric V. Smith]

Eric V. Smith  added the comment:

Ah, good point. I agree the example should make that clear. And I think a note 
in .remove() about using it while iterating would be a good idea, too.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Stefan Behnel]

Stefan Behnel  added the comment:

> That example is especially problematic.

No, it's not. It uses .findall(), which returns a list. It's like when you make 
a copy of a list to iterate over, when you want to modify the original list in 
the loop.

That could be made explicit in the text that introduces the example, but I 
think it's a very good example.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Serhiy Storchaka]

Change by Serhiy Storchaka :


--
assignee:  -> docs@python
components: +Documentation
nosy: +docs@python, eli.bendersky, scoder

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Eric V. Smith]

Eric V. Smith  added the comment:

I think the only action here is to improve the documentation. That example is 
especially problematic.

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

You will get the same behavior for lists:

>>> a = [1, 2, 3]
>>> for x in a:
... if x == 1:
... a.remove(x)
... print(x)
... 
1
3

Lists are iterated by index. First you get an item at index 0, then at index 1, 
etc, to the end of the list. Initially the list is [1, 2, 3]. After removing 1 
at the first iteration it becomes [2, 3], and at the next iteration you get an 
item at index 1 which is 3.

--
nosy: +serhiy.storchaka

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[Eric V. Smith]

Eric V. Smith  added the comment:

I assume that ElementTree doesn't support mutation while iterating.

However, the docs at 
https://docs.python.org/3/library/xml.etree.elementtree.html#modifying-an-xml-file
 show removing an item while iterating. It probably only works because the 
child being removed is the last one.

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nosy: +eric.smith

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[issue41892] use both "for in" and "ElementTree.remove" has a index bug

[WoodyWoo]

Change by WoodyWoo :


--
title: use both "for" and "ElementTree.remove" has a index bug -> use both "for 
in" and "ElementTree.remove" has a index bug

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