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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"sqlite3: Blob crashes when using a negative-step slice","articleBody":"### What happened?\n\n`sqlite3.Blob` crashes when you use a negative step in a slice — both for reading and writing. \nThe [sequence operations docs](https://docs.python.org/3/library/stdtypes.html#typesseq-common) say `s[i:j:k]` with negative `k` is valid, and `bytearray` handles it fine.\n\n```python\nimport sqlite3\n\ncon = sqlite3.connect(\":memory:\")\ncon.execute(\"CREATE TABLE t(b BLOB)\")\ndata = b\"this blob data string is exactly fifty bytes long!\"\ncon.execute(\"INSERT INTO t(b) VALUES (?)\", (data,))\n\n# bytearray works\nba = bytearray(data)\nba[9:0:-2] = b\"12345\"\nprint(ba)\n# b't5i4 3l2b1data string is exactly fifty bytes long!'\n\n# Blob crashes\nblob = con.blobopen(\"t\", \"b\", 1)\nblob[9:0:-2] = b\"12345\"\nblob.close()\n```\n\n```\n\u003e\u003e\u003e blob[9:0:-2] = b\"12345\"\nTraceback (most recent call last):\n  File \"\u003cstdin\u003e\", line 1, in \u003cmodule\u003e\nSystemError: Negative size passed to PyBytes_FromStringAndSize\n```\n\nReading also crashes: `blob[9:0:-2]` raises the same error.\n\nOn Python 3.11.6 and 3.14.5 the error is `SystemError: Negative size passed to PyBytes_FromStringAndSize`. \nOn the current main branch `read_multiple` was changed to use `PyBytesWriter_Create`, so the message is now `ValueError: size must be \u003e= 0` — but the root cause is the same.\n\n### CPython versions tested on:\n\n3.11.6, 3.14.5, main(629da5c914b4407e01c1dc06cbcbd8dce825fef3)\n\n### Operating systems tested on:\n\nmacOS - 26.3.1 (a)(25D771280a)\n\n---\n\n### Related\n\n- gh-142787 / gh-142824 — different bug (assertion failure on empty slice, already fixed)\n- https://docs.python.org/3/library/stdtypes.html#typesseq-common (`s[i:j:k]`, negative k)\n\n\n---\nI'd like to work on a fix for this. I'll submit a PR.\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-150450\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ever0de","@type":"Person","name":"ever0de"},"datePublished":"2026-05-26T05:07:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/150449/cpython/issues/150449"}