Previously, assigning an out-of-range integer (negative or > 255) or an integer too large for i64 (e.g. 2**65) to a Blob index raised OverflowError instead of ValueError.

Mirror CPython's ass_subscript_index logic:

• Convert the value via to_i64(), treating any overflow as -1
• Validate the result is in [0, 255], raising ValueError("byte must be in range(0, 256)") otherwise
• Separate deletion error messages: "item deletion" for index, "slice deletion" for slice

Ref: https://github.com/python/cpython/blob/main/Modules/_sqlite/blob.c

Problem

When assigning an out-of-range integer to a Blob index, RustPython raised OverflowError instead of ValueError:

blob[0] = -1      # OverflowError: Python int too large to convert to Rust u8
blob[0] = 256     # OverflowError
blob[0] = 2**65   # OverflowError

CPython raises ValueError: byte must be in range(0, 256) for all three cases.

Root Cause

The previous implementation used try_to_primitive::<u8>(vm)? to convert the assigned value, which calls Rust's type conversion and raises a generic OverflowError for any value that doesn't fit in u8 — including negative numbers.

Fix

Mirrors CPython's ass_subscript_index logic:

// CPython blob.c
long val = PyLong_AsLong(value);
if (val == -1 && PyErr_Occurred()) {
    PyErr_Clear();
    val = -1;  // treat overflow as out-of-range
}
if (val < 0 || val > 255) {
    PyErr_SetString(PyExc_ValueError, "byte must be in range(0, 256)");
    return -1;
}

In Rust:

• Replace try_to_primitive::<u8>() with as_bigint().to_i64().unwrap_or(-1)
• Values that overflow i64 (e.g. 2**65) are treated as -1, which then fails the range check
• Raises ValueError("byte must be in range(0, 256)") for any value outside [0, 255]

Additionally, aligned the deletion error messages with CPython:

• del blob[0] → "Blob doesn't support item deletion"
• del blob[5:10] → "Blob doesn't support slice deletion"

Changes

• _sqlite3.rs: Fix Blob::ass_subscript index branch
• test_dbapi.py: Remove @unittest.expectedFailure from test_blob_set_item_error

Summary by CodeRabbit

• Bug Fixes

  • Fixed single-byte BLOB assignment to properly validate byte values and handle integer overflow.
  • Improved BLOB slice assignment so unaffected bytes are preserved for complex slices (including negative-step writes).

• Tests

  • Added a regression test for extended-slice BLOB assignments to prevent future regressions.