On Sunday, May 24, 2020, at 08:07 -0400, Steven D'Aprano wrote:

> On Sun, May 24, 2020 at 02:27:00PM +0300, Ram Rachum wrote:
>
>> Today I wrote a script and did this:
>>
>>     sorted(paths, key=lambda path: len(str(path)), reverse=True)
>>
>> But it would have been nicer if I could do this:
>>
>>     sorted(paths, key=len, reverse=True)
>
> It would have been even nicer if we could compose functions:
>
>     sorted(paths, key=len∘str, reverse=True)
>
> *semi-wink*

It started with y = len(str(f(g(h(x))))), which is ugly.  Some people
like pipes, and wrote object-like functions that could be composed with
the "|" character slash:

    y = x | h | g | f | str | len

(Or something like that.  Maybe there was a ">" at the beginning.)  Then
a clojure fan I know showed me function called "thread":

    f = thread([len, str, f, g, h])
    y = f(x)

An untested Python implementation:

    def thread(fs):  # or maybe you like def thread(*fs) instead
        fs = reversed(fs) # or not, depending on how fs was constructed
        def inner(x):
            for f in fs:
                x = f(x)
            return x
        return inner

    sorted(paths, key=thread([len, str]), reverse=True)

On the one hand, it's not quite as concise as composing the functions
directly.  On the other hand, ∘ ruffles a lot of ASCII feathers (but I'm
sure Steven knows that).
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