[Python-Dev] PEP 467: Minor API improvements to bytes, bytearray, and memoryview

[Serhiy Storchaka]

On 08.06.16 14:26, Paul Sokolovsky wrote:
> On Wed, 8 Jun 2016 14:05:19 +0300
> Serhiy Storchaka <storchaka at gmail.com> wrote:
>
>> On 08.06.16 13:37, Paul Sokolovsky wrote:
>>>> The obvious way to create the bytes object of length n is b'\0' *
>>>> n.
>>>
>>> That's very inefficient: it requires allocating useless b'\0', then
>>> a generic function to repeat arbitrary memory block N times. If
>>> there's a talk of Python to not be laughed at for being SLOW, there
>>> would rather be efficient ways to deal with blocks of binary data.
>>
>> Do you have any evidences for this claim?
>
> Yes, it's written above, let me repeat it: bytes(n) is (can be)
> calloc(1, n) underlyingly, while b"\0" * n is a more complex algorithm.
>
>>
>> $ ./python -m timeit -s 'n = 10000' -- 'bytes(n)'
>> 1000000 loops, best of 3: 1.32 usec per loop
>> $ ./python -m timeit -s 'n = 10000' -- 'b"\0" * n'
>> 1000000 loops, best of 3: 0.858 usec per loop
>
> I don't know how inefficient CPython's bytes(n) or how efficient
> repetition (maybe 1-byte repetitions are optimized into memset()?), but
> MicroPython (where bytes(n) is truly calloc(n)) gives expected results:
>
> $ ./run-bench-tests bench/bytealloc*
> bench/bytealloc:
>      3.333s (+00.00%) bench/bytealloc-1-bytes_n.py
>      11.244s (+237.35%) bench/bytealloc-2-repeat.py

If the performance of creating an immutable array of n zero bytes is 
important in MicroPython, it is worth to optimize b"\0" * n.

For now CPython is the main implementation of Python 3 and bytes(n) is 
slower than b"\0" * n in CPython.