What for -- for? (was A bug?)

Joshua Landau joshua at landau.ws
Thu Oct 30 18:21:56 CET 2014

On 29 October 2014 03:22, Rustom Mody <rustompmody at gmail.com> wrote:
> Yesterday I was trying to introduce python to some senior computer scientists.
> Tried showing a comprehension-based dir-walker vs a for-loop based one:
> def dw(p):
>    if isfile(p):
>       return [p]
>    else:
>       return [p] + [c for f in listdir(p) for c in dw(p+'/'+f)]
> Comment to me : "Well this is neat and compact, but it does not add
> anything fundamental (over usual index based for-loops)"
> I tried to say that 'for' over general sequences is quite different
> and significantly more powerful than C/Pascal for over indexes +
> explicit indexing.

If you really want to show the generality of iteration, I suggest you
start with iterators:

    def walk(path):
        yield path

        if isdir(path):
            for name in iterdir(path):
                for file in walk(path + "/" + name):
                    yield file

This is fundementally inexpressable with indexes. It also lends itself
to expressing delegation (eg. "yield from walk(path + "/" + name)").

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