sqlite3: Blob crashes when using a negative-step slice

What happened?

sqlite3.Blob crashes when you use a negative step in a slice — both for reading and writing.
The sequence operations docs say s[i:j:k] with negative k is valid, and bytearray handles it fine.

import sqlite3

con = sqlite3.connect(":memory:")
con.execute("CREATE TABLE t(b BLOB)")
data = b"this blob data string is exactly fifty bytes long!"
con.execute("INSERT INTO t(b) VALUES (?)", (data,))

# bytearray works
ba = bytearray(data)
ba[9:0:-2] = b"12345"
print(ba)
# b't5i4 3l2b1data string is exactly fifty bytes long!'

# Blob crashes
blob = con.blobopen("t", "b", 1)
blob[9:0:-2] = b"12345"
blob.close()

>>> blob[9:0:-2] = b"12345"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: Negative size passed to PyBytes_FromStringAndSize

Reading also crashes: blob[9:0:-2] raises the same error.

On Python 3.11.6 and 3.14.5 the error is SystemError: Negative size passed to PyBytes_FromStringAndSize.
On the current main branch read_multiple was changed to use PyBytesWriter_Create, so the message is now ValueError: size must be >= 0 — but the root cause is the same.

CPython versions tested on:

3.11.6, 3.14.5, main(629da5c914b4407e01c1dc06cbcbd8dce825fef3)

Operating systems tested on:

macOS - 26.3.1 (a)(25D771280a)

Related

• gh-142787 / gh-142824 — different bug (assertion failure on empty slice, already fixed)
• https://docs.python.org/3/library/stdtypes.html#typesseq-common (s[i:j:k], negative k)

I'd like to work on a fix for this. I'll submit a PR.

Linked PRs

• gh-150450
• gh-155773

Start with the sqlite3.Blob slicing implementation and reproduce the reported negative-step cases for both reading and writing using the example in the issue. Done means negative-step slices behave like the documented sequence operations without raising a size-related error; linked PRs gh-150450 and gh-155773 show that work is already underway.