Python import coding style

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The (previously) top-voted answer to this question is nicely formatted but absolutely wrong about performance. Let me demonstrate

Performance

Top Import

import random

def f():
    L = []
    for i in xrange(1000):
        L.append(random.random())


for i in xrange(1000):
    f()

$ time python import.py

real        0m0.721s
user        0m0.412s
sys         0m0.020s

Import in Function Body

def f():
    import random
    L = []
    for i in xrange(1000):
        L.append(random.random())

for i in xrange(1000):
    f()

$ time python import2.py

real        0m0.661s
user        0m0.404s
sys         0m0.008s

As you can see, it can be more efficient to import the module in the function. The reason for this is simple. It moves the reference from a global reference to a local reference. This means that, for CPython at least, the compiler will emit LOAD_FAST instructions instead of LOAD_GLOBAL instructions. These are, as the name implies, faster. The other answerer artificially inflated the performance hit of looking in sys.modules by importing on every single iteration of the loop.

As a rule, it’s best to import at the top but performance is not the reason if you are accessing the module a lot of times. The reasons are that one can keep track of what a module depends on more easily and that doing so is consistent with most of the rest of the Python universe.

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