CODEWITHSUNDEEP
pythonpython-2.7
user2242044
user2242044

Reputation: 9213

Which method is faster for Python iterating?

I am creating a new list from a large old list based on a certain value being not equal to None. Which method of iteration is faster?

Option 1:

new_list = []
for values in old_list:
   if values[4] is not None:
       new_list.append(values[4])

Option 2:

new_list = [x for x in old_list if x[4] is not None]

Upvotes: 1

Views: 114

Answers (2)

Jivan
Jivan

Reputation: 23058

Try to timeit both.

But second is widely known to be faster.

Basically map is faster than list comprehension which is faster than for loop.

A whole lot of literature is available on the web about this subject.

EDIT:

I promised an update with actual, tangible results. Here is the code.

import random
import timeit

old_list = [ random.randint(0, 100000) for i in range(0, 100) ]


def floop(old_list):
    new_list = []
    for value in old_list:
        new_list.append(value)
    return new_list


def lcomp(old_list):
    new_list = [ value for value in old_list ]
    return new_list


if __name__=='__main__':
    results_floop = timeit.Timer('floop(old_list)', "from __main__ import floop, old_list").timeit()
    results_lcomp = timeit.Timer('lcomp(old_list)', "from __main__ import lcomp, old_list").timeit()
    print("Function\t\tSeconds elapsed")
    print("For loop\t\t{}".format(results_floop))
    print("List comp\t\t{}".format(results_lcomp))

Remember: timeit loops through the called function 1 million times and prints the time elapsed in seconds. Read it like to execute this 1 million times, it took xx seconds.

Here are the results. I think they speak by themselves.

~/python » python3 lists.py
Function        Seconds elapsed
For loop        11.089475459069945
List comp       5.985794545034878

Upvotes: 5

rlms
rlms

Reputation: 11060

The second is both faster, and more readable. If you need more speed, and are only iterating through the result once, you could use filter - new_list = filter(lambda x: x[4] is not None, old_list). You could call list on the filtered result, but that might not have any speed advantage over the list comprehension (and is less Pythonic IMHO).

Upvotes: 1

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