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To: python-list@python.org
From: "Frank Millman" <frank@chagford.com>
Subject: Re: Dict comprehensions - improvement to docs?
Date: Tue, 17 Mar 2015 08:44:30 +0200
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"Ian Kelly" <ian.g.kelly@gmail.com> wrote in message 
news:CALwzid=u19YMkfJBhbLZi1qh2U9UK4ohY5wco1zO-i3T5AtyOA@mail.gmail.com...
> 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

Thanks for the explanation. I'll try not to make that mistake again.

However, to go back to the original example, we want to compare a dict 
comprehension with a dict() constructor using a generator expression.

Let's see if I have got this one right -

C:\>python -m timeit -s "x=range(65, 91); y=[chr(z) for z in x]" "dict((a, 
b) for a, b in zip(x, y))"
10000 loops, best of 3: 49.6 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)}"
10000 loops, best of 3: 25.8 usec per loop

Or to use Paul's original example -

C:\>python -m timeit "d = dict((k, v) for k, v in [('name', 'paul'), 
('language', 'python')])
100000 loops, best of 3: 16.6 usec per loop

C:\>python -m timeit "d = {k: v for k, v in [('name', 'paul'), ('language', 
'python')]}
100000 loops, best of 3: 5.2 usec per loop

It seems that a dict comp is noticeably faster.

Does this sound right, or are there other factors I should be taking into 
account?

Frank