#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.

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.

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.

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

> 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.

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.

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.

Only that makes "for each in list" literally.

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.

@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.

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.

@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'''<?xml version="1.0"?>
<data>
    <country name="Liechtenstein">
        <rank updated="yes">2</rank>
        <year>2008</year>
        <gdppc>141100</gdppc>
        <neighbor name="Austria" direction="E"/>
        <neighbor name="Switzerland" direction="W"/>
    </country>
    <country name="Singapore">
        <rank updated="yes">5</rank>
        <year>2011</year>
        <gdppc>59900</gdppc>
        <neighbor name="Malaysia" direction="N"/>
    </country>
    <country name="Panama1">
        <rank updated="yes">69</rank>
        <year>2011</year>
        <gdppc>13600</gdppc>
        <neighbor name="Costa Rica" direction="W"/>
        <neighbor name="Colombia" direction="E"/>
    </country>
    <country name="Panama2">
        <rank updated="yes">69</rank>
        <year>2011</year>
        <gdppc>13600</gdppc>
        <neighbor name="Costa Rica" direction="W"/>
        <neighbor name="Colombia" direction="E"/>
    </country>
</data>'''
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 <count:
    rank = int (root[index].find("rank").text)
    if rank>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")

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

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.

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

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

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

Closing since this works as designed.

Users are responsible for avoiding concurrent tree modifications during iteration.