Converting a list of strings into a list of integers?

Jan Riechers janpeterr at freenet.de
Sun Jul 22 19:27:29 CEST 2012


On 22.07.2012 20:01, Steven D'Aprano wrote:
[SNIP]
> map is faster than an ordinary for-loop if the function you are applying
> is a builtin like int, str, etc. But if you have to write your own pure-
> Python function, the overhead of calling a function negates the advantage
> of map, which is no faster than a for-loop. For example:
>
> results = map(int, sequence)  # calls builtin `int`
>
> hoists the call to int into the fast C layer, instead of the slow Python
> layer, and should be faster than
>
> results = []
> for x in sequence:
>      results.append(int(x))
>
> which runs at the speed of Python. But:
>
> results = map(lambda x: x+1, sequence)  # calls pure Python function
>
> if no faster than a for-loop:
>
> results = []
> for x in sequence:
>      results.append(x+1)
>
> Note: this has*nothing*  to do with the use of lambda. Writing the "+1"
> function above using def instead of lambda would give the same results.
[SNAP]

Hi Steven,

besides that I testdrive Pypy (and still am impressed, other topic) - 
your answer was what I was picking for ;)

Especially this part of you:
 > map is faster than an ordinary for-loop if the function you are applying
 > is a builtin like int, str, etc. [underlaying c-layer] But if you 
have to write your own pure-
 > Python function, the overhead of calling a function negates the advantage
 > of map [...]

I did not know that the speed gain is up foremost present when using 
built-ins, but that's for sure something to keep in mind when writing code.

Thanks for your explanation, clarifies a lot!

Jan



More information about the Python-list mailing list