Efficient ways to duplicate array/list in Python

Question

Note: I\'m a Ruby developer trying to find my way in Python.

When I wanted to figure out why some scripts use mylist[:] instead of list(mylist)

Answer 1

Use the timeit module in python for testing timings.

from copy import *

a=range(1000)

def cop():
    b=copy(a)

def func1():
    b=list(a)

def slice():
    b=a[:]

def slice_len():
    b=a[0:len(a)]



if __name__=="__main__":
    import timeit
    print "copy(a)",timeit.timeit("cop()", setup="from __main__ import cop")
    print "list(a)",timeit.timeit("func1()", setup="from __main__ import func1")
    print "a[:]",timeit.timeit("slice()", setup="from __main__ import slice")
    print "a[0:len(a)]",timeit.timeit("slice_len()", setup="from __main__ import slice_len")

Results:

copy(a) 3.98940896988
list(a) 2.54542589188
a[:] 1.96630120277                   #winner
a[0:len(a)] 10.5431251526

It's surely the extra steps involved in a[0:len(a)] are the reason for it's slowness.

Here's the byte code comparison of the two:

In [19]: dis.dis(func1)
  2           0 LOAD_GLOBAL              0 (range)
              3 LOAD_CONST               1 (100000)
              6 CALL_FUNCTION            1
              9 STORE_FAST               0 (a)

  3          12 LOAD_FAST                0 (a)
             15 SLICE+0             
             16 STORE_FAST               1 (b)
             19 LOAD_CONST               0 (None)
             22 RETURN_VALUE        

In [20]: dis.dis(func2)
  2           0 LOAD_GLOBAL              0 (range)
              3 LOAD_CONST               1 (100000)
              6 CALL_FUNCTION            1
              9 STORE_FAST               0 (a)

  3          12 LOAD_FAST                0 (a)    #same up to here
             15 LOAD_CONST               2 (0)    #loads 0
             18 LOAD_GLOBAL              1 (len) # loads the builtin len(),
                                                 # so it might take some lookup time
             21 LOAD_FAST                0 (a)
             24 CALL_FUNCTION            1         
             27 SLICE+3             
             28 STORE_FAST               1 (b)
             31 LOAD_CONST               0 (None)
             34 RETURN_VALUE