Issue 29649: struct.pack_into check boundary error message ignores offset

For this situation, check boundary error message didn't correctly show out.

>>> import struct
>>> import ctypes
>>> byte_list = ctypes.create_string_buffer(1)
>>> struct.pack_into('b', byte_list, 5, 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: pack_into requires a buffer of at least 1 bytes

Since offset is setting at 5, it should at least need `offset + soself->s_size` bytes to store it.

Thanks! Just added PR fixing this.

I'm not sure the new error message is better. It is not known what is wrong -- the size of the buffer, or the offset?

I think that the better error message should include three numbers: the offset, the size of the destination buffer, and the size of packed data or the minimal size of the destination buffer.

yeah, I also thought about this too.
Something like 'pack_into requires a buffer of at least 6 bytes (size is 1, offset is 5)'

Or "pack_into requires a buffer of at least 6 bytes for packing 1 bytes at offset 5".

thanks Serhiy! just implemented your variant in my PR.

Different error messages are needed if original offset < 0. For example packing 4 bytes with offset -2 always fails, not depending of the size of the buffer. Packing into buffer of size 10 with offset -11 always fails, not depending of the size of packed data.

struct.pack_into('<I', bytearray(10), -2, 123)
struct.pack_into('<B', bytearray(10), -11, 123)

Thanks Serhiy! Just implemented new error messages in my PR for case you mentioned.

any updates on this issue? looks like PR is ready to go

New changeset f78b119364b521307237a1484ba5f43f42300898 by Serhiy Storchaka (Andrew Nester) in branch 'master':
bpo-29649: Improve struct.pack_into() boundary error messages (#424)
https://github.com/python/cpython/commit/f78b119364b521307237a1484ba5f43f42300898

Thank you for your contribution Andrew.

stage: patch review -> resolved