Is the defaultdict in Python's collections module really faster than using setdefault?

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情书的邮戳
情书的邮戳 2021-02-04 04:07

I\'ve seen other Python programmers use defaultdict from the collections module for the following use case:

from collections import defaultdict

s = [(\'yellow\'         


        
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  •  醉梦人生
    2021-02-04 04:44

    Yes, there is something "wrong":

    You have put the creation of the (default)dict into the statement instead of the setup. Constructing a new defaultdict is more expensive than a normal dict, and usually that's not the bottleneck you should be profiling in a program - after all, you build your data structures once but you use them many times.

    If you do your tests like below, you see that defaultdict operations are indeed faster:

    >>> import timeit
    >>> setup1 = """from collections import defaultdict
    ... s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
    ... d = defaultdict(list)"""
    >>> stmt1 = """for k, v in s:
    ...     d[k].append(v)"""
    >>> setup2 = """s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
    ... d = {}"""
    >>> stmt2 = """for k, v in s:
    ...     d.setdefault(k, []).append(v)"""
    >>> timeit.timeit(setup=setup1, stmt=stmt1)
    1.0283400125194078
    >>> timeit.timeit(setup=setup2, stmt=stmt2)
    1.7767367580925395
    

    Python 2.7.3 on Win7 x64.

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