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

Chris Barker chris.barker at noaa.gov
Fri Feb 23 14:23:04 EST 2018 
On Fri, Feb 23, 2018 at 10:45 AM, Guido van Rossum <guido at python.org> wrote:

> There are useful things you can only do with comprehensions if the second
> for-loop can use the variable in the first for-loop. E.g.
>
> [(i, j) for i in range(10) for j in range(i)]
>

indeed -- and that is fairly common use-case in nested for loops -- so good
to preserve this.

But I still think the original:

[g(y) for x in range(5) for y in [f(x)]]

Is always going to be confusing to read. Though I do agree that it's not
too bad when you unpack it into for loops:

In [89]: for x in range(5):
    ...:     for y in [f(x)]:
    ...:         l.append(g(y))

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),)]

-CHB

-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
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Chris.Barker at noaa.gov
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