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[Python-Dev] The `for y in [x]` idiom in comprehensions

Stefan Behnel stefan_ml at behnel.de
Fri Feb 23 15:50:39 EST 2018 
Chris Barker schrieb am 23.02.2018 um 20:23:
> BTW, would it be even a tiny bit more efficient to use a tuple in the inner
> loop?
> 
> [g(y) for x in range(5) for y in (f(x),)]

Serhiy's optimisation does not use a loop at all anymore and folds it into
a direct assignment "y=f(x)" instead.

But in general, yes, changing a list iterable into a tuple is an
improvement as tuples are more efficient to allocate. Haven't tried it in
CPython (*), but it might make a slight difference for very short
iterables, which are probably common. Although the execution of the loop
body will likely dominate the initial allocation by far.

Stefan



(*) I implemented this list->tuple transformation in Cython a while ago,
but seeing Serhiy's change now got me thinking that this could be further
improved into a stack allocated C array, to let the C compiler unroll the
loop at will. I'll probably try that at some point...

https://github.com/cython/cython/issues/2117
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