passing memoryview slice to struct packing functions

[danielhrisca]

BPO	42408
Nosy	@danielhrisca

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created_at = <Date 2020-11-19.13:41:25.178>
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title = 'passing memoryview slice to struct packing functions'
updated_at = <Date 2020-11-19.13:41:25.178>
user = 'https://github.com/danielhrisca'

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issue_num = 42408
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url = 'https://bugs.python.org/issue42408'
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[danielhrisca]

First I have to apologize if this was discussed before; I've searched the issues and googled the problem but I got nothing obvious.

I'm trying to pass a memoryview slice as argument to struct packing functions to avoid creating intermediate bytes objects

from struct import Struct
s = Struct('<2Q500s').pack_into
buffer = bytearray(7000)
data = bytes(300)
s(buffer, 0, 500, 500, memoryview(data)[4:])

and I get this error:
error: argument for 's' must be a bytes object

Why is it not possible to use memoryviews in this case?

It is also strange for me that we can use a memoyview as the target buffer, so this works fine:

s(memoryview(buffer)[10:], 0, 500, 500, data[4:])