gh-150449: Fix sqlite3.Blob crash with negative-step slices

Doc/library/sqlite3.rst

@@ -1760,6 +1760,10 @@ Blob objects
 
    .. versionadded:: 3.11
 
+   .. versionchanged:: next
+      :class:`Blob` now supports negative-step slices
+      (e.g. ``blob[9:0:-2]``) for both reading and writing.
+
    A :class:`Blob` instance is a :term:`file-like object`
    that can read and write data in an SQLite :abbr:`BLOB (Binary Large OBject)`.
    Call :func:`len(blob) <len>` to get the size (number of bytes) of the blob.

Doc/whatsnew/3.16.rst

@@ -116,6 +116,14 @@ shlex
   a string, even if it is already safe for a shell without being quoted.
   (Contributed by Jay Berry in :gh:`148846`.)
 
+sqlite3
+-------
+
+* :class:`sqlite3.Blob` now supports negative-step slices for reading and
+  writing (e.g. ``blob[9:0:-2]``).  Previously, such slices would raise
+  :exc:`SystemError` or :exc:`ValueError`.
+  (Contributed by Jiseok CHOI in :gh:`150449`.)
+
 xml
 ---
 

Lib/test/test_sqlite3/test_dbapi.py

@@ -1390,6 +1390,16 @@ def test_blob_get_slice_negative_index(self):
     def test_blob_get_slice_with_skip(self):
         self.assertEqual(self.blob[0:10:2], b"ti lb")
 
+    def test_blob_get_slice_with_negative_step(self):
+        # gh-150449: negative-step slices must not crash
+        self.assertEqual(self.blob[9:0:-2], self.data[9:0:-2])
+        self.assertEqual(self.blob[9::-2], self.data[9::-2])
+        self.assertEqual(self.blob[::-1], self.data[::-1])
+        # When start <= stop with a negative step the slice is empty; this
+        # must return b"" rather than crashing or raising an exception.
+        self.assertEqual(self.blob[3:8:-1], self.data[3:8:-1])   # b""
+        self.assertEqual(self.blob[5:5:-1], self.data[5:5:-1])   # b""
+
     def test_blob_set_slice(self):
         self.blob[0:5] = b"12345"
         expected = b"12345" + self.data[5:]
@@ -1418,6 +1428,27 @@ def test_blob_set_slice_with_skip(self):
         expected = b"1h2s3b4o5 " + self.data[10:]
         self.assertEqual(actual, expected)
 
+    def test_blob_set_slice_with_negative_step(self):
+        # gh-150449: negative-step slice assignment must not crash
+        expected = bytearray(self.data)
+        expected[9:0:-2] = b"12345"
+        self.blob[9:0:-2] = b"12345"
+        actual = self.cx.execute("select b from test").fetchone()[0]
+        self.assertEqual(actual, bytes(expected))
+
+        # Also verify a slice that includes index 0
+        expected2 = bytearray(self.data)
+        expected2[9::-2] = b"12345"
+        self.blob[9::-2] = b"12345"
+        actual2 = self.cx.execute("select b from test").fetchone()[0]
+        self.assertEqual(actual2, bytes(expected2))
+
+        # When start <= stop with a negative step the slice is empty;
+        # assigning b"" to it must be a no-op (blob contents unchanged).
+        state_before = bytes(self.blob[:])
+        self.blob[3:8:-1] = b""
+        self.assertEqual(bytes(self.blob[:]), state_before)
+
     def test_blob_mapping_invalid_index_type(self):
         msg = "indices must be integers"
         with self.assertRaisesRegex(TypeError, msg):

Misc/NEWS.d/next/Library/2026-05-26-15-53-50.gh-issue-150449.GfDWxl.rst

@@ -0,0 +1,3 @@
+:class:`sqlite3.Blob` now supports negative-step slices for reading and
+writing (e.g. ``blob[9:0:-2]``).  Previously, such slices would raise
+:exc:`SystemError` or :exc:`ValueError`.

Modules/_sqlite/blob.c

@@ -445,7 +445,14 @@ subscript_slice(pysqlite_Blob *self, PyObject *item)
         return read_multiple(self, len, start);
     }
 
-    PyObject *blob = read_multiple(self, stop - start, start);
+    // Compute the contiguous blob region covering all slice elements, then
+    // copy each element using the standard size_t-cursor pattern that handles
+    // both positive and negative steps via unsigned arithmetic.
+    Py_ssize_t last = start + (len - 1) * step;
+    Py_ssize_t read_offset = Py_MIN(start, last);
+    Py_ssize_t read_length = Py_ABS(start - last) + 1;
+
+    PyObject *blob = read_multiple(self, read_length, read_offset);
     if (blob == NULL) {
         return NULL;
     }
@@ -456,10 +463,12 @@ subscript_slice(pysqlite_Blob *self, PyObject *item)
         return NULL;
     }
     char *res_buf = PyBytesWriter_GetData(writer);
-
     char *blob_buf = PyBytes_AS_STRING(blob);
-    for (Py_ssize_t i = 0, j = 0; i < len; i++, j += step) {
-        res_buf[i] = blob_buf[j];
+
+    size_t cur;
+    Py_ssize_t i;
+    for (cur = (size_t)start, i = 0; i < len; cur += (size_t)step, i++) {
+        res_buf[i] = blob_buf[(Py_ssize_t)cur - read_offset];
     }
     Py_DECREF(blob);
     return PyBytesWriter_Finish(writer);
@@ -553,13 +562,23 @@ ass_subscript_slice(pysqlite_Blob *self, PyObject *item, PyObject *value)
         rc = inner_write(self, vbuf.buf, len, start);
     }
     else {
-        PyObject *blob_bytes = read_multiple(self, stop - start, start);
+        // Compute the contiguous blob region covering all slice elements, then
+        // update each element using the standard size_t-cursor pattern that
+        // handles both positive and negative steps via unsigned arithmetic.
+        Py_ssize_t last = start + (len - 1) * step;
+        Py_ssize_t write_offset = Py_MIN(start, last);
+        Py_ssize_t write_length = Py_ABS(start - last) + 1;
+        PyObject *blob_bytes = read_multiple(self, write_length, write_offset);
         if (blob_bytes != NULL) {
             char *blob_buf = PyBytes_AS_STRING(blob_bytes);
-            for (Py_ssize_t i = 0, j = 0; i < len; i++, j += step) {
-                blob_buf[j] = ((char *)vbuf.buf)[i];
+            size_t cur;
+            Py_ssize_t i;
+            for (cur = (size_t)start, i = 0; i < len;
+                 cur += (size_t)step, i++) {
+                blob_buf[(Py_ssize_t)cur - write_offset] =
+                    ((char *)vbuf.buf)[i];
             }
-            rc = inner_write(self, blob_buf, stop - start, start);
+            rc = inner_write(self, blob_buf, write_length, write_offset);
             Py_DECREF(blob_bytes);
         }
     }