[Python-ideas] proposed methods: list.replace / list.indices

[Nick Coghlan]

On Mon, Dec 31, 2012 at 12:10 AM, Ned Batchelder <ned at nedbatchelder.com> wrote:
> The two sides (count/index and replace/indexes) seem about the same to me:
>
> - They are unambiguous operations.  That is, no one has objected that
> reasonable people might disagree about how .replace() should behave, which
> is a common reason not to add things to the stdlib.
> - They implement simple operations that are easy to explain and will find
> use.  In my experience, .indexes() is at least as useful as .count().
> - All are based on element equality semantics.
> - Any of them could be implemented in a few lines of Python.
>
> What is the organizing principle for the methods list (or any other built-in
> data structure) should have? I would hate for the main criterion to be,
> "these are the methods that existed in Python 2.3," for example.   Why is
> .count() in and .replace() out?

The general problem with adding new methods to types rather than
adding new functions+protocols is that it breaks ducktyping. We can
mitigate that now by adding the new methods to
collections.abc.Sequence, but it remains the case that relying on
these methods being present rather than using the functional
equivalent will needlessly couple your code to the underlying sequence
implementation (since not all sequences inherit from the ABC, some are
just registered).

We also have a problem with replace() specifically that it *does*
already exist in the standard library, as a non-mutating operation on
str, bytes and bytearray. Adding it as a mutating method on sequences
in general would create an immediate name conflict in the bytearray
method namespace. That alone is a dealbreaker for that part of the
idea.

The question of an "indices" builtin or itertools function is
potentially more interesting, but really, I don't think the algorithm
David noted in his original post rises to the level of needing
standardisation or acceleration:

    def indices(seq, val):
        for i, x in enumerate(seq):
            if x == val: yield i

    def map_assign(store, keys, val):
        for k in keys:
            store[k] = val

    def replace(seq, old, new):
        map_assign(seq, indices(seq, old), new)

    seq = [x, a, y, a]
    replace(seq, a, b)
    assert seq == [x, b, y, b]

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia