On Fri, Feb 21, 2020 at 02:42:16PM +0200, Serhiy Storchaka wrote:
> 21.02.20 10:36, Steven D'Aprano пише:
> >On my machine, at least, constructing a bytes object first followed by
> >an array is significantly faster than the alternative:
> >
> >[steve@ando cpython]$ ./python -m timeit -s "from array import array"
> >"array('i', bytes(500000))"
> >100 loops, best of 5: 1.71 msec per loop
> >
> >[steve@ando cpython]$ ./python -m timeit -s "from array import array"
> >"array('i', [0])*500000"
> >50 loops, best of 5: 7.48 msec per loop
> >
> >That surprises me and I cannot explain it.
>
> The second one allocates and copies 4 times more memory.
I completely misunderstood what the first would do. I expected it to
create an array of 500,000 zeroes, but it only created an array of
125,000. That's nuts! The docstring says:
Return a new array whose items are restricted by typecode, and
initialized from the optional initializer value, which must be a
list, string or iterable over elements of the appropriate type.
A bytes object is an iterable over integers, so I expected these two to
be equivalent:
array('i', bytes(500000))
array('i', [0]*500000)
I never would have predicted that a bytes iterable and a list iterable
behave differently. Oh, you have to read the documentation on the
website:
https://docs.python.org/3/library/array.html#array.array
Okay, let's try that again:
[steve@ando cpython]$ ./python -m timeit -s "from array import array"
"array('i', bytes(500000*4))"
20 loops, best of 5: 12.6 msec per loop
compared to 7.65 milliseconds for the version using multiplication. That
makes more sense to me.
Okay, I'm starting to come around to giving array an alternate
constructor:
array.zeroes(typecode, size [, *, value=None])
If keyword-only argument value is given and is not None, it is used as
the initial value instead of zero.
--
Steven
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