On Fri, Jan 08, 2021 at 11:19:40PM +0200, Ram Rachum wrote:
> Today I had a need: I had a tuple of dynamic sequence-like objects. I
> wanted to iterate on them reversed, starting with items of the last one and
> slowly making my way towards the first one.
>
> In short, I want `reversed(itertools.chain(x, y, z))` that behaves like
> `itertools.chain(map(reversed, (z, y, x)))`.
That would break backwards compatibility, because
`reversed(itertools.chain(x, y, z))` is already possible today, and it
does *not* behave in that fashion.
reversed reverses whatever iterable it is given, it doesn't single out
chain objects for special magical handling. If I write this:
reversed([None, 'abc', 'def', 'ghi'])
I expect to get
'ghi', 'def', 'abc', None
and not
'i', 'h', 'g', 'f', 'e', 'd', 'c', 'b', 'a', raise TypeError
If I replace the list with itertools.chain, I should still get exactly
the same results, and I do. Breaking backwards compatibility for your
special case is not going to happen.
> What do you think?
I think you should just write `itertools.chain(map(reversed, (z, y, x)))`.
If you don't have the individual x, y, z sequences, but only their
tuple t = (x, y, z), you have two choices:
itertools.chain(map(reversed, t[::-1]))
itertools.chain(map(reversed, reversed(t)))
They have slightly different meanings, so you get to choose whichever
one suits your use-case better.
Not every trivial combination of functions needs to be given a built-in
or standard library solution. Especially not if doing so will break
backwards compatibility.
"I had three numbers in a tuple, and wanted half of twice the first
number added to the difference of the remaining two. What do you think
about making `len((a,b,c))` return `(2*a + abs(b - c))/2`?"
*wink*
--
Steve
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