[Python-ideas] Length hinting and preallocation for container types

[Ethan Furman]

On 03/05/2013 11:44 AM, Eli Bendersky wrote:
>
>     >    The real problem is that this code is not idiomatic Python, especially if
>     >    you want it to be reasonably fast:
>     >
>     >    � lst = []
>     >    � for i in range(100):
>     >    � � � lst.append(i*i)
>     >
>     >    Why not:
>     >
>     >    lst = [i*i for i in range(100)]
>     >
>     >    If the "append" pattern is complex, just "preallocate" like this:
>     >
>     >    lst = [0] * 100
>     >
>     >    And then fill it.
>     >
>     >    Eli
>
>     >How would you replicate the behavior of __exit__ in Christian's example?
>
>
> Why would I want to replicate it?

I suspect the new behavior would be most useful when you don't know precisely how large the final list will be: 
overallocate (possibly by a large margin), then __exit__ returns the unused portion back to the pool).

--
~Ethan~