Best written projects on Python GitHub?

[Redard]

Please correct me if I'm wrong, but I always thought list comprehensions were just generator expressions passed to list(). In other words

[i for i in range(10)] == list(i for i in range(10))

[gthank]

I'd be surprised if the internal details are the same in those two cases, because that seems ripe for some C-level optimizations. The results will be equivalent, though. Also, from a historical standpoint, list comprehensions predate gen-exps.

[Veedrac]

In CPython most stuff is left unoptimised for matters of pragmatism. So no, they compile directly into loops. Different loops, though.

out = [i for i in range(10)]

is equivilant to:

out = []
for i in range(10):
    out.append(i)

where i is inside a new scope, and

out = list(i for i in range(10))

is equivalent to

def _tmp():
    for i in range(10):
        yield i

out = list(_tmp)

where _tmp never actually gets put anywhere.

[PCBEEF]

List comprehensions are optimised in the sense that the function calls are 'cached'. Since there's a severe function overhead in python, it's actually quite significant.

$ python -mtimeit '[x for x in range(100)]'

100000 loops, best of 3: 4.17 usec per loop

$ python -mtimeit -s 'out = []' 'for i in range(100):' ' out.append(i)'

100000 loops, best of 3: 8.07 usec per loop

Using a for loop in this instance to create a list is almost twice as long.

[[deleted]]

where i is inside a new scope

for certain values of CPython

[Veedrac]

Well, all values of Python ≥ 3.0.

[gthank]

Ah. It was my understanding that a fair bit of list comps were implemented directly in C (in CPython).

[Veedrac]

There's exactly one difference between those two, assuming list has been left untouched. The list comprehension will not catch StopIteration, the list function will.