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

[David Kreuter]

Hi python-ideas.

I think it would be nice to have a method in 'list' to replace certain
elements by others in-place. Like this:

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

The alternatives are longer than they should be, imo. For example:

    for i, n in enumerate(l):
        if n == a:
            l[i] = b

Or:

    l = [b if n==a else n for n in l]

And this is what happens when someone tries to "optimize" this process.
It totally obscures the intention:

    try:
        i = 0
        while i < len(l):
            i = l.index(a, i)
            l[i] = b
            i += 1
    except ValueError:
        pass

If there is a reason not to add '.replace' as built-in method, it could be
implemented in pure python efficiently if python provided a version of
'.index' that returns the index of more than just the first occurrence of a
given item. Like this:

    l = [x, a, b, a]
    for i in l.indices(a):
        l[i] = b

So adding .replace and/or .indices… Good idea? Bad idea?
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