Strings beginning with underscore not removed from lists - feature or bug?

[yemiteliyadu]

BPO	33157
Nosy	@stevendaprano, @zhangyangyu

^{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 2018-03-27.15:24:21.910>
created_at = <Date 2018-03-27.15:15:51.054>
labels = ['type-bug', 'invalid']
title = 'Strings beginning with underscore not removed from lists - feature or bug?'
updated_at = <Date 2018-03-27.15:33:44.520>
user = 'https://bugs.python.org/yemiteliyadu'

bugs.python.org fields:

activity = <Date 2018-03-27.15:33:44.520>
actor = 'steven.daprano'
assignee = 'none'
closed = True
closed_date = <Date 2018-03-27.15:24:21.910>
closer = 'xiang.zhang'
components = []
creation = <Date 2018-03-27.15:15:51.054>
creator = 'yemiteliyadu'
dependencies = []
files = []
hgrepos = []
issue_num = 33157
keywords = []
message_count = 4.0
messages = ['314530', '314531', '314532', '314533']
nosy_count = 3.0
nosy_names = ['steven.daprano', 'xiang.zhang', 'yemiteliyadu']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33157'
versions = ['Python 3.6']

[yemiteliyadu]

Strings beginning with underscore not removed from lists
Reproducible as shown below:

Python 3.6.4 |Anaconda custom (64-bit)| (default, Jan 16 2018, 12:04:33) 
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> test = ["a","_a","b","_b"]
>>> for i in test: print(i)
... 
a
_a
b
_b
>>> for i in test: test.remove(i)
... 
>>> test
['_a', '_b']
>>> 

Is this a feature or a bug?
A search through the docs did not show any mention of this.

[zhangyangyu]

It's just the right behavior. You are modifying the list while iterating over it. It has no business of underscore.

>>> test = ['_a', 'a', '_b', 'b']
>>> for i in test: test.remove(i)
>>> test
['a', 'b']

[zhangyangyu]

You may refer to stackoverflow for an explanation, for example, https://stackoverflow.com/questions/6260089/strange-result-when-removing-item-from-a-list

[stevendaprano]

In addition to Xiang Zhang's comments, this is neither a feature nor a bug, but a misunderstanding. This has nothing to do with strings or underscores:

py> L = [1, 2, 3, 4, 5]
py> for item in L:
... L.remove(item)
...
py> L
[2, 4]

When you modify a list as you iterate over it, the results can be unexpected. Don't do it.

If you *must* modify a list that you are iterating over, you must do so backwards, so that you are only removing items from the end, not the beginning of the list:

py> L = [1, 2, 3, 4, 5]
py> for i in range(len(L)-1, -1, -1):
... L.remove(L[i])
...
py> L
[]

But don't do that: it is nearly always must faster to make a copy of the list containing only the items you wish to keep, then assign back to the list using slicing. A list comprehension makes this an easy one-liner:

py> L = [1, 2, 3, 4, 5]
py> L[:] = [x for x in L if x > 4]
py> L
[5]