[issue45271] Add a 'find' method (a'la str) to list

[Raymond Hettinger]

Raymond Hettinger  added the comment:

This is one bit of symmetry that we shouldn't pursue.  The find() methods have 
been a recurring source of bugs because of failing to check for a -1 result but 
having that be a valid index into the sequence.

Also, the sequence APIs are long established.  If find() were really needed, we 
would have felt the pain and added it long ago.

As Steve says, you can take this to python-ideas, but my recommendation is to 
not do it all.

--
nosy: +rhettinger
resolution:  -> rejected
stage:  -> resolved
status: open -> closed

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[issue45271] Add a 'find' method (a'la str) to list

[Steve Dower]

Steve Dower  added the comment:

Strings are already special in that str.index() and str.find() both find 
substrings, while list.index() only finds a single element.

If .find() also searched for a substring of the list, I think this could be 
helpful. Even more so if it used an efficient algorithm (bearing in mind that 
the arbitrary comparisons between elements - something else that doesn't exist 
in strings - would make this complicated).

This is probably something to bring up on the python-ideas mailing list first, 
anyway. Symmetry is not a sufficient reason in itself to add new APIs - often 
the asymmetry exists because the "missing" one is only there for 
legacy/compatibility reasons, not because it is a good API.

--
nosy: +steve.dower

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[issue45271] Add a 'find' method (a'la str) to list

[Dávid Nemeskey]

Change by Dávid Nemeskey :


--
type:  -> enhancement

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[issue45271] Add a 'find' method (a'la str) to list

[Dávid Nemeskey]

New submission from Dávid Nemeskey :

There is an unjustified asymmetry between `str` and `list`, as far as lookup 
goes. Both have an `index()` method that returns the first index of a value, or 
raises a `ValueError` if it doesn't exist. However, only `str` has the `find` 
method, which returns -1 if the value is not in the string.

I think it would make sense to add `find` to `list` as well. For starters, it 
would make the API between the two sequence types more consistent. More 
importantly (though it depends on the use-case), `find` is usually more 
convenient than `index`, as one doesn't have to worry about handling an 
exception. As a bonus, since `list` is mutable, it allows one to write code 
such as

if (idx := lst.find(value)) == -1:
lst.append(value)
call_some_function(lst[idx])

, making the method even more useful as it is in `str`.

--
messages: 402497
nosy: nemeskeyd
priority: normal
severity: normal
status: open
title: Add a 'find' method (a'la str) to list

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