Answer by River for List changes unexpectedly after assignment. How do I clone or copy it to prevent this?

Python 3.6 Timings

Here are the timing results using Python 3.6.8. Keep in mind these times are relative to one another, not absolute.

I stuck to only doing shallow copies, and also added some new methods that weren't possible in Python2, such as list.copy() (the Python3 slice equivalent) and two forms of list unpacking (*new_list, = list and new_list = [*list]):

METHOD                  TIME TAKENb = [*a]                2.75180600000021b = a * 1               3.50215399999990b = a[:]                3.78278899999986  # Python2 winner (see above)b = a.copy()            4.20556500000020  # Python3 "slice equivalent" (see above)b = []; b.extend(a)     4.68069800000012b = a[0:len(a)]         6.84498999999959*b, = a                 7.54031799999984b = list(a)             7.75815899999997b = [i for i in a]      18.4886440000000b = copy.copy(a)        18.8254879999999b = []for item in a:  b.append(item)        35.4729199999997

We can see the Python2 winner still does well, but doesn't edge out Python3 list.copy() by much, especially considering the superior readability of the latter.

The dark horse is the unpacking and repacking method (b = [*a]), which is ~25% faster than raw slicing, and more than twice as fast as the other unpacking method (*b, = a).

b = a * 1 also does surprisingly well.

Note that these methods do not output equivalent results for any input other than lists. They all work for sliceable objects, a few work for any iterable, but only copy.copy() works for more general Python objects.

Here is the testing code for interested parties (Template from here):

import timeitCOUNT = 50000000print("Array duplicating. Tests run", COUNT, "times")setup = 'a = [0,1,2,3,4,5,6,7,8,9]; import copy'print("b = list(a)\t\t", timeit.timeit(stmt='b = list(a)', setup=setup, number=COUNT))print("b = copy.copy(a)\t", timeit.timeit(stmt='b = copy.copy(a)', setup=setup, number=COUNT))print("b = a.copy()\t\t", timeit.timeit(stmt='b = a.copy()', setup=setup, number=COUNT))print("b = a[:]\t\t", timeit.timeit(stmt='b = a[:]', setup=setup, number=COUNT))print("b = a[0:len(a)]\t\t", timeit.timeit(stmt='b = a[0:len(a)]', setup=setup, number=COUNT))print("*b, = a\t\t\t", timeit.timeit(stmt='*b, = a', setup=setup, number=COUNT))print("b = []; b.extend(a)\t", timeit.timeit(stmt='b = []; b.extend(a)', setup=setup, number=COUNT))print("b = []; for item in a: b.append(item)\t", timeit.timeit(stmt='b = []\nfor item in a:  b.append(item)', setup=setup, number=COUNT))print("b = [i for i in a]\t", timeit.timeit(stmt='b = [i for i in a]', setup=setup, number=COUNT))print("b = [*a]\t\t", timeit.timeit(stmt='b = [*a]', setup=setup, number=COUNT))print("b = a * 1\t\t", timeit.timeit(stmt='b = a * 1', setup=setup, number=COUNT))