python:loop vs comprehension

This question already has an answer here:

Are list-comprehensions and functional functions faster than “for loops”? 5 answers

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You can use the timeit library, or just use time.time() to time it yourself:

>>> from time import time
>>> def first():
...     ftime = time()
...     _foo = [x ** 2 for x in range(1, 101)]
...     print "First", time()-ftime
... 
>>> def second():
...     ftime = time()
...     _foo = []
...     for x in range(1, 101):
...             _b=[x**2]
...             _foo+=_b
...     print "Second", time()-ftime
... 
>>> first()
First 5.60283660889e-05
>>> second()
Second 8.79764556885e-05
>>> first()
First 4.88758087158e-05
>>> second()
Second 8.39233398438e-05
>>> first()
First 2.8133392334e-05
>>> second()
Second 7.29560852051e-05
>>> 

Evidently, the list comprehension runs faster, by a factor of around 2 to 3.

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