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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"sqlite3: Blob slice assignment skips validation for empty slices","articleBody":"# Bug report\n\n### Bug description:\n\n`sqlite3.Blob` slice assignment skips type and size validation when the\ntarget slice is empty (`start == stop`, or any slice whose computed\nlength is zero).\n\nAs a result, assignments that would normally raise `TypeError` or\n`IndexError` complete silently when the target slice is empty.\n\nThis behavior is inconsistent with non-empty `sqlite3.Blob` slice\nassignment and appears to be caused by an early return in\n`ass_subscript_slice()` before validation is performed.\n\n### Reproduction\n\n```python\nimport sqlite3\n\ncon = sqlite3.connect(\":memory:\")\ncon.execute(\"CREATE TABLE t(b BLOB)\")\ncon.execute(\"INSERT INTO t VALUES(zeroblob(10))\")\ncon.commit()\n\nblob = con.blobopen(\"t\", \"b\", 1)\n\n# Non-empty slice — expected behavior\nblob[0:3] = b\"12345\"  # IndexError\nblob[0:3] = None      # TypeError\n\n# Empty slice — BUG\nblob[5:5] = b\"123\"    # silently succeeds\nblob[5:5] = None      # silently succeeds\n```\n\n### Expected behavior\n\nEmpty slice assignments should be validated in the same way as non-empty\nslice assignments.\n\n```python\nblob[5:5] = b\"123\"\n```\n\nshould raise:\n\n```text\nIndexError: Blob slice assignment is wrong size\n```\n\nand\n\n```python\nblob[5:5] = None\n```\n\nshould raise:\n\n```text\nTypeError: a bytes-like object is required, not 'NoneType'\n```\n\n### Actual behavior\n\nBoth assignments complete successfully. The blob remains unchanged and\nno exception is raised.\n\n### Root cause\n\nIn `Modules/_sqlite/blob.c`, `ass_subscript_slice()` returns early when\nthe computed slice length is zero.\n\nBecause this early return occurs before the bytes-like object check and\nslice-size validation, invalid assignments are silently accepted for\nempty slices.\n\n### Related\n\nDiscovered during review of #150450 (negative-step slice handling for\n`sqlite3.Blob`).\n\nReview discussion:\n\nhttps://github.com/python/cpython/pull/150450#discussion_r3326381267\n\n### CPython versions tested on:\n\n3.14\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-150915\n* gh-150923\n* gh-150924\n* gh-150925\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ever0de","@type":"Person","name":"ever0de"},"datePublished":"2026-06-04T12:16:34.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/150913/cpython/issues/150913"}