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pythonlistperformancecopy

Which method to copy a flat list is faster: comprehension, slice, or copy.copy?

Say I want to make a shallow copy of a list in python. Which method is fastest?

I can think of

  • copy.copy(l)
  • l[:]
  • [x for x in l]
  • list(l)

This question is specifically about the relative speed of making a shallow copy of a list in Python. Specifically, I am interested in CPython.

Solution

  • Tested in jupyter notebook, python 3.8

    l = list(range(10000))
%%timeit
[x for x in l]
# 175 µs ± 5.23 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

%%timeit
copy.copy(l)
# 22.6 µs ± 365 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

%%timeit
l[:]
# 22 µs ± 1.28 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

%%timeit
list(l)
# 21.6 µs ± 558 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

    So they're all the same except the list comprehension, which is far slower.