Dict comprehensions - improvement to docs?

[Frank Millman]

"Ian Kelly" <ian.g.kelly at gmail.com> wrote in message 
news:CALwzidmNDcSvER7S6inEaVZA=DHUrDX1KzL-WRVwhd=o3_LvWA at mail.gmail.com...
> On Tue, Mar 17, 2015 at 12:44 AM, Frank Millman <frank at chagford.com> 
> wrote:
>> 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
>
> Why did you revert back to the no-op generator expression in the first
> case instead of the more efficient dict(zip(x, y))?

Firstly, I just want to emphasise that I am not trying to prove anything 
here, I am trying to learn something.

Why do you call it a no-op? I understood your previous point that a 
generator in the setup statement is exhausted after the first execution, but 
this is in the run-time statement, so I thought it would be executed every 
time.

If one does not need any 'comprehension' functionality, I agree that 
dict(zip(x, y)) is the way to go.

However, if you do need a comprehension, Paul questioned why a special dict 
comprehension was introduced when you can easily use a generator expression.

I ran the timing tests to compare the two, and I was surprised to see such a 
difference. However, you have already pointed out one case where I was not 
comparing apples and apples, and it is quite possible that I have stumbled 
across another one.

I read Lele's post where he pointed out that part of the difference is 
explained by the fact that dict() involves a function call, whereas {} does 
not. However, this does not seem to explain the entire difference.

Frank