struct.pack (and pack_into / Struct.pack) only accepted bytes and bytearray for the s and p format codes. Passing any other buffer — memoryview, array.array, mmap, ... — failed with struct.error, even though it's a contiguous chunk of bytes that can be copied straight in:

>>> struct.pack("4s", memoryview(b"abad"))
struct.error: argument for 's' must be a bytes object

Both codes now accept any contiguous bytes-like object. bytes/bytearray keep their existing fast path; anything else goes through the buffer protocol (PyObject_GetBuffer with PyBUF_SIMPLE), and the buffer is released on every exit path. Non-contiguous buffers and objects that don't expose a buffer still raise struct.error (with a slightly clearer message now).

serhiy-storchaka said as much on the issue — "there is no reason to limit it to bytes and bytearray, any object providing continuous buffer can be supported".

Added a regression test covering memoryview and array.array through pack, pack_into and Struct.pack, and updated the docs, which until now spelled out the bytes/bytearray-only restriction.

• Issue: struct.pack with format s errors on bytes-like data #148529