Why is dictionary.keys() a list and not a set?

[Fredrik Lundh]

bonono at gmail.com wrote:

> I know that is a single list of tuples, I mean that can be used as
> well.
>
> for k, _ in d.items(): print k
> for _, v in d.items(): print v
> for k in d.keys(): print k
> for v in d.values(): print v
>
> Would there be a noticeable performance difference ?

Sloppy use of print statements is a great way to produce misleading
benchmarks [1], since the time it takes to print lots of stuff tends to
overshadow the actual algorithm (for bonus points, print to a terminal
window, and use "time" to measure performance, so you get startup
times as well).  If you don't necessarily want to print all the stuff, my
original assertion still holds:

    "creating a list full of tuples just to pull them apart and throw
    them all away isn't exactly the most efficient way to do things"

Consider a dictionary with one million items.  The following operations

    k = d.keys()
    v = d.values()

creates two list objects, while

    i = d.items()

creates just over one million objects.  In your "equivalent" example,
you're calling d.items() twice to produce two million objects, none
of which you really care about.

</F>

1) careful use of timeit and generator expressions is another one:
http://online.effbot.org/2005_01_01_archive.htm#20050125 (code)
http://online.effbot.org/2005_01_01_archive.htm#20050127 (explanation)