list extend vs nested loop

list_alloc.py

"""
>>> a = [[1,2], [3,3], [4]]
>>> o = []
>>> for x in a:
...     o.extend(x)
...
>>> o
[1, 2, 3, 3, 4]


>>> [y for x in a for y in x]
[1, 2, 3, 3, 4]
>>>
"""

import time

# Creates 10,000 buckets each contains 1000 integers
random_array = [ [x for x in xrange(0, 1000)] for x in xrange(0, 10000) ]
output = []
t1 = time.time()
for x in random_array:
    output.extend(x)
t2 = time.time()
print(t2-t1)

t3 = time.time()
[y for sub_array in random_array for y in sub_array]
t4 = time.time()
print(t4-t3)


"""
ywong:tmp ywong$ python p.py
0.0799720287323
0.858078956604
ywong:tmp ywong$ python p.py
0.0888578891754
0.858464002609
ywong:tmp ywong$ python p.py
0.0784840583801
0.870872974396

Increase the size of each bucket to 10,000
ywong:tmp ywong$ python p.py
0.796358108521
9.03610396385
"""