Friday, 2 June 2017

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



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



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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