`from ... import ...` has a significant performance overhead

[Dreamsorcerer]

Bug report

Maybe there's some fundamental reason I'm not aware of, but using the from syntax when importing creates variables which are typically over 4x slower to reference for some reason.

Expected behaviour

I'd expect these 2 versions of code to produce basically the same result functionally and performance-wise:

from random import sample

import random
sample = random.sample
del random

Actual results

The first one is over 5x slower to reference:

> python3 -m timeit 'from random import sample' 'sample'
500000 loops, best of 5: 823 nsec per loop
> python3 -m timeit 'import random' 'random.sample'
2000000 loops, best of 5: 161 nsec per loop
> python3 -m timeit 'import random; sample = random.sample' 'sample'
2000000 loops, best of 5: 161 nsec per loop

CPython versions tested on:

3.9 and 3.12

Operating systems tested on:

Linux

[Dreamsorcerer]

Each nested level of import seems to have an increasing impact too, and reassigning to another variable has no impact:

> python3 -m timeit 'from aiohttp.web import Response' 'Response'
500000 loops, best of 5: 827 nsec per loop
> python3 -m timeit 'from aiohttp.web import Response; r = Response' 'r'
500000 loops, best of 5: 851 nsec per loop
> python3 -m timeit 'from aiohttp import web; Response = web.Response' 'Response'
500000 loops, best of 5: 691 nsec per loop
> python3 -m timeit 'import aiohttp.web; Response = aiohttp.web.Response' 'Response'
1000000 loops, best of 5: 394 nsec per loop

[qierenrongku]

python -m timeit --setup='import random' 'random.sample'
20000000 loops, best of 5: 10.7 nsec per loop
python -m timeit 'import random' 'random.sample'
5000000 loops, best of 5: 85.9 nsec per loop
python -m timeit 'from random import sample' 'sample'
2000000 loops, best of 5: 127 nsec per loop
python -m timeit 'import random'
5000000 loops, best of 5: 70.2 nsec per loop

If import random has already been imported, it will be obtained from the cache instead of being imported again, so it takes less time.

[zware]

Note that multiple arguments to timeit are treated as multiple lines in the tested script, not as setup followed by test (use -s or --setup for setup statements). Using python3 -m timeit -s 'from random import sample' 'sample' and python3 -m timeit -s 'import random; sample = random.sample' 'sample' makes the difference go away completely. The from import itself is somewhat less performant than import, as expected because it does more work.

[Dreamsorcerer]

Damn, I've been using timeit wrong for ages...