[] and {} vs list() and dict(), which is better?

python performance list dictionary
Noah McIlraith picture Noah McIlraith · Apr 26, 2011 · Viewed 123.3k times · Source

I understand that they are both essentially the same thing, but in terms of style, which is the better (more Pythonic) one to use to create an empty list or dict?

Answer

Greg Haskins picture Greg Haskins · Apr 26, 2011

In terms of speed, it's no competition for empty lists/dicts:

>>> from timeit import timeit
>>> timeit("[]")
0.040084982867934334
>>> timeit("list()")
0.17704233359267718
>>> timeit("{}")
0.033620194745424214
>>> timeit("dict()")
0.1821558326547077

and for non-empty:

>>> timeit("[1,2,3]")
0.24316302770330367
>>> timeit("list((1,2,3))")
0.44744206316727286
>>> timeit("list(foo)", setup="foo=(1,2,3)")
0.446036018543964
>>> timeit("{'a':1, 'b':2, 'c':3}")
0.20868602015059423
>>> timeit("dict(a=1, b=2, c=3)")
0.47635635255323905
>>> timeit("dict(bar)", setup="bar=[('a', 1), ('b', 2), ('c', 3)]")
0.9028228448029267

Also, using the bracket notation lets you use list and dictionary comprehensions, which may be reason enough.

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