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

[WoodyWoo]

BPO	41892
Nosy	@scoder, @ericvsmith, @ned-deily, @serhiy-storchaka, @miss-islington
PRs	bpo-41892: Clarify an example in the ElementTree docs #22464 bpo-41892: Clarify an example in the ElementTree docs #22546 [3.8] bpo-41892: Clarify that an example in the ElementTree docs explicitly avoids modifying an XML tree while iterating over it. (GH-22464) #22553 [3.9] bpo-41892: Clarify that an example in the ElementTree docs explicitly avoids modifying an XML tree while iterating over it. (GH-22464) #22554

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-10-05.04:22:13.395>
created_at = <Date 2020-09-30.14:20:25.049>
labels = ['type-bug', '3.8', '3.9', '3.10', 'expert-XML', 'invalid', 'docs']
title = 'use both "for in" and "ElementTree.remove" has a index bug'
updated_at = <Date 2020-10-05.04:22:13.395>
user = 'https://bugs.python.org/WoodyWoo'

bugs.python.org fields:

activity = <Date 2020-10-05.04:22:13.395>
actor = 'scoder'
assignee = 'docs@python'
closed = True
closed_date = <Date 2020-10-05.04:22:13.395>
closer = 'scoder'
components = ['Documentation', 'XML']
creation = <Date 2020-09-30.14:20:25.049>
creator = 'WoodyWoo'
dependencies = []
files = []
hgrepos = []
issue_num = 41892
keywords = ['patch']
message_count = 18.0
messages = ['377698', '377699', '377700', '377701', '377702', '377703', '377704', '377705', '377706', '377707', '377724', '377732', '377756', '377764', '377982', '377984', '377989', '377992']
nosy_count = 8.0
nosy_names = ['scoder', 'eric.smith', 'ned.deily', 'eli.bendersky', 'docs@python', 'serhiy.storchaka', 'miss-islington', 'WoodyWoo']
pr_nums = ['22464', '22546', '22553', '22554']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41892'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[WoodyWoo]

#Just run it in Python v3.8.6 of win7 32bit 
import xml.etree.ElementTree as ET 
xmlstr='''<root>
<a no="1"/>
<b/>
<c/>
</root>'''
etroot = ET.fromstring(xmlstr)
for ELEMchild in etroot:
    if ELEMchild.get("no") == "1" :
        etroot.remove(ELEMchild)       #so far so good
    print (ELEMchild.tag)
    #It should be :  "b /n c" or "a /n b /n c",I can live with it both.
    #But it is :  "a /n c".
    #The index of ELEMchild should not +1 when there was a remove method worked on one of the before ELEMs.
    #I was forced to use while and if to control the index +1/+0.
    #BTW,ELEM has no method returning index in siblings, which is buging me too.

[ericvsmith]

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.

[serhiy-storchaka]

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.

[ericvsmith]

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