Delete element in a list that do not meet certain conditions using a loop

Question

While reading a list 'lst', I want to remove an delement that does not meet certain condition. Based on the following answer: Strange result when removing item from a list while iterating over it I found a solution that works great. Here is the code in PyCharm:

for ind, el in enumerate(lst):
    if not el.strip() or el.strip() != 'LABEL':
        lst[ind] = None #  here I get a warning concerning ind
    else:
        break
lst = [n for n in lst if n is not None]

I cannot figure out why I receive this warning:

Unexpected type(s): (int, None) Possible type(s): (SupportsIndex, str) (slice, Iterable[str]) 
Inspection info:
Reports type errors in function call expressions, targets, and return values. In a dynamically typed language, this is possible in a limited number of cases.
Types of function parameters can be specified in docstrings or in Python 3 function annotations.

Also, the `not el.strip()` part seems redundant, and why do you `break` after the first valid element? And why not do it all in a list comp in the first place? — tobias_k, Sep 17 '21 at 07:32
@tobias_k I believe they want to keep the first `"LABEL"` and everything that follows it. — Selcuk, Sep 17 '21 at 07:35

Selcuk · Accepted Answer · 2021-09-17T07:36:18.933

3

We don't see the rest of the code but you probably type-hinted your lst to have str elements only and None is not a str.

You don't have to implement a 2-pass algorithm to remove elements though. The following should be equivalent to your code, sans warnings (see docs for dropwhile):

from itertools import dropwhile

lst = list(dropwhile(lambda el: el.strip() != 'LABEL', lst))

edited Sep 17 '21 at 07:36

answered Sep 17 '21 at 07:30

Selcuk

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score -1 · Answer 2 · answered Sep 17 '21 at 07:45

-1

You can use list comprehension and do it all on one line. See the code below:

new_list = [x for x in lst if x.strip() != 'LABEL']

One more thing, I don't it is a good idea to modify a list while looping through it.

answered Sep 17 '21 at 07:45

Daniel Afriyie

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This won't produce the same output as the OPs code (note the `break` statement in the original). Also, `I don't it is a good idea to modify a list while looping through it`: Why not? That's exactly what `list`s are for. That's why they are mutable while many other data structures in Python are not. – Selcuk Sep 17 '21 at 07:47
With modifying a list while looping through it you can read the answer that was given to the question the OP included in his question. Here is the link https://stackoverflow.com/a/6260097/11979729 – Daniel Afriyie Sep 17 '21 at 08:06
@DanielAfriyie Note the difference between "modifying" and "adding/deleting elements". Just editing or even replacing elements while you loop the list is usually fine. – tobias_k Sep 17 '21 at 09:40
@tobias_k Ok. I got it. – Daniel Afriyie Sep 17 '21 at 13:02

Delete element in a list that do not meet certain conditions using a loop

2 Answers2