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Author josh.r
Recipients josh.r
Date 2016-01-20.18:07:38
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Marked as misclassified Yes
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copy.copy uses a relatively high overhead approach to copying list, set and dict, using:

    def _copy_with_constructor(x):
        return type(x)(x)

This is despite the fact that all three types implement a .copy() method, and there is already a defined method:

    def _copy_with_copy_method(x):
        return x.copy()

that (in %timeit tests) runs with substantially less overhead, in percentage terms; for empty lists, sets and dicts, calling _copy_with_constructor or _copy_with_copy_method directly on them, the times on my machine (Python 3.5.0, Linux x86-64) are:

empty list: 281 ns (constructor), 137 ns (method)
empty set: 275 ns (constructor), 175 ns (method)
empty dict: 372 ns (constructor), 211 ns (method)

The method costs could be trimmed further if _copy_with_copy_method was changed from a Python implementation to using operator.methodcaller, e.g.

        # If we have _operator, avoids cost of importing Python code; it's part of core modules in CPython, already loaded for free
        from _operator import methodcaller  
    except ImportError:
        from operator import methodcaller

    _copy_with_copy_method = methodcaller('copy')

This doesn't save a whole lot more (shaves another 9-17 ns off the times for the Python version of _copy_with_copy_method), but it's nice in that it avoids reinventing the wheel in the copy module. Combining the two changes (to use methodcaller for _copy_with_copy_method and to have list, set and dict use _copy_with_copy_method) means we can get rid of both Python defined functions in favor of a single use of operator.methodcaller used by all types that previously used either of them.

Obviously not a high priority fix, but I noticed this while trying to figure out a way to fix zlib's lack of support in the copy module ( #26166 which apparently duplicates #25007 ) and how to work around it.
Date User Action Args
2016-01-20 18:07:38josh.rsetrecipients: + josh.r
2016-01-20 18:07:38josh.rsetmessageid: <>
2016-01-20 18:07:38josh.rlinkissue26167 messages
2016-01-20 18:07:38josh.rcreate