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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Mon, 16 Mar 2015 09:57:07 -0600
Subject: Re: Dict comprehensions - improvement to docs?
To: Python <python-list@python.org>
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On Mon, Mar 16, 2015 at 3:01 AM, Frank Millman <frank@chagford.com> wrote:
> C:\>python -m timeit -s "x = range(65, 91); y = (chr(z) for z in x)"
> "dict(zip(x, y))"
> 100000 loops, best of 3: 11.9 usec per loop
>
> C:\>python -m timeit -s "x = range(65, 91); y = (chr(z) for z in x)" "{a: b
> for a, b in zip(x, y)}"
> 100000 loops, best of 3: 7.24 usec per loop

Since the setup code is only run once, the generator expression used
for y is only iterated over once. On every subsequent loop, zip is
producing an empty result. So this measurement is really just
capturing the overhead of the dict construction. Compare:

$ python3 -m timeit -s "x = range(65, 91); y = (chr(z) for z in x)"
"dict(zip(x,y))"
1000000 loops, best of 3: 0.9 usec per loop
$ python3 -m timeit -s "x = range(65, 91); y = [chr(z) for z in x]"
"dict(zip(x,y))"
100000 loops, best of 3: 2.69 usec per loop
$ python3 -m timeit -s "x = range(65, 91); y = (chr(z) for z in x)"
"{a:b for a,b in zip(x,y)}"
1000000 loops, best of 3: 0.837 usec per loop
$ python3 -m timeit -s "x = range(65, 91); y = [chr(z) for z in x]"
"{a:b for a,b in zip(x,y)}"
100000 loops, best of 3: 2.67 usec per loop