gh-69093: Add context manager support to sqlite3.Blob (GH-91562)

Doc/includes/sqlite3/blob.py

@@ -4,9 +4,13 @@ con = sqlite3.connect(":memory:")
 con.execute("create table test(blob_col blob)")
 con.execute("insert into test(blob_col) values (zeroblob(10))")
 
-blob = con.blobopen("test", "blob_col", 1)
-blob.write(b"Hello")
-blob.write(b"World")
-blob.seek(0)
-print(blob.read())  # will print b"HelloWorld"
-blob.close()
+# Write to our blob, using two write operations:
+with con.blobopen("test", "blob_col", 1) as blob:
+    blob.write(b"Hello")
+    blob.write(b"World")
+
+# Read the contents of our blob
+with con.blobopen("test", "blob_col", 1) as blob:
+    greeting = blob.read()
+
+print(greeting)  # outputs "b'HelloWorld'"

Doc/library/sqlite3.rst

@@ -1115,6 +1115,11 @@ Blob Objects
    data in an SQLite :abbr:`BLOB (Binary Large OBject)`.  Call ``len(blob)`` to
    get the size (number of bytes) of the blob.
 
+   Use the :class:`Blob` as a :term:`context manager` to ensure that the blob
+   handle is closed after use.
+
+   .. literalinclude:: ../includes/sqlite3/blob.py
+
    .. method:: close()
 
       Close the blob.
@@ -1149,10 +1154,6 @@ Blob Objects
       current position) and :data:`os.SEEK_END` (seek relative to the blob’s
       end).
 
-   :class:`Blob` example:
-
-      .. literalinclude:: ../includes/sqlite3/blob.py
-
 
 .. _sqlite3-types:
 

Lib/test/test_sqlite3/test_dbapi.py

@@ -1170,6 +1170,25 @@ class BlobTests(unittest.TestCase):
         with self.assertRaises(TypeError):
             b"a" in self.blob
 
+    def test_blob_context_manager(self):
+        data = b"a" * 50
+        with self.cx.blobopen("test", "b", 1) as blob:
+            blob.write(data)
+        actual = self.cx.execute("select b from test").fetchone()[0]
+        self.assertEqual(actual, data)
+
+        # Check that __exit__ closed the blob
+        with self.assertRaisesRegex(sqlite.ProgrammingError, "closed blob"):
+            blob.read()
+
+    def test_blob_context_manager_reraise_exceptions(self):
+        class DummyException(Exception):
+            pass
+        with self.assertRaisesRegex(DummyException, "reraised"):
+            with self.cx.blobopen("test", "b", 1) as blob:
+                raise DummyException("reraised")
+
+
     def test_blob_closed(self):
         with memory_database() as cx:
             cx.execute("create table test(b blob)")
@@ -1186,6 +1205,10 @@ class BlobTests(unittest.TestCase):
                 blob.seek(0)
             with self.assertRaisesRegex(sqlite.ProgrammingError, msg):
                 blob.tell()
+            with self.assertRaisesRegex(sqlite.ProgrammingError, msg):
+                blob.__enter__()
+            with self.assertRaisesRegex(sqlite.ProgrammingError, msg):
+                blob.__exit__(None, None, None)
 
     def test_blob_closed_db_read(self):
         with memory_database() as cx:

Misc/NEWS.d/next/Library/2022-04-14-00-59-01.gh-issue-69093.bmlMwI.rst

@@ -0,0 +1,2 @@
+Add :term:`context manager` support to :class:`sqlite3.Blob`.
+Patch by Aviv Palivoda and Erlend E. Aasland.

Modules/_sqlite/blob.c

@@ -307,8 +307,51 @@ blob_tell_impl(pysqlite_Blob *self)
 }
 
 
+/*[clinic input]
+_sqlite3.Blob.__enter__ as blob_enter
+
+Blob context manager enter.
+[clinic start generated code]*/
+
+static PyObject *
+blob_enter_impl(pysqlite_Blob *self)
+/*[clinic end generated code: output=4fd32484b071a6cd input=fe4842c3c582d5a7]*/
+{
+    if (!check_blob(self)) {
+        return NULL;
+    }
+    return Py_NewRef(self);
+}
+
+
+/*[clinic input]
+_sqlite3.Blob.__exit__ as blob_exit
+
+    type: object
+    val: object
+    tb: object
+    /
+
+Blob context manager exit.
+[clinic start generated code]*/
+
+static PyObject *
+blob_exit_impl(pysqlite_Blob *self, PyObject *type, PyObject *val,
+               PyObject *tb)
+/*[clinic end generated code: output=fc86ceeb2b68c7b2 input=575d9ecea205f35f]*/
+{
+    if (!check_blob(self)) {
+        return NULL;
+    }
+    close_blob(self);
+    Py_RETURN_FALSE;
+}
+
+
 static PyMethodDef blob_methods[] = {
     BLOB_CLOSE_METHODDEF
+    BLOB_ENTER_METHODDEF
+    BLOB_EXIT_METHODDEF
     BLOB_READ_METHODDEF
     BLOB_SEEK_METHODDEF
     BLOB_TELL_METHODDEF

Modules/_sqlite/clinic/blob.c.h

@@ -162,4 +162,55 @@ blob_tell(pysqlite_Blob *self, PyObject *Py_UNUSED(ignored))
 {
     return blob_tell_impl(self);
 }
-/*[clinic end generated code: output=d3a02b127f2cfa58 input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(blob_enter__doc__,
+"__enter__($self, /)\n"
+"--\n"
+"\n"
+"Blob context manager enter.");
+
+#define BLOB_ENTER_METHODDEF    \
+    {"__enter__", (PyCFunction)blob_enter, METH_NOARGS, blob_enter__doc__},
+
+static PyObject *
+blob_enter_impl(pysqlite_Blob *self);
+
+static PyObject *
+blob_enter(pysqlite_Blob *self, PyObject *Py_UNUSED(ignored))
+{
+    return blob_enter_impl(self);
+}
+
+PyDoc_STRVAR(blob_exit__doc__,
+"__exit__($self, type, val, tb, /)\n"
+"--\n"
+"\n"
+"Blob context manager exit.");
+
+#define BLOB_EXIT_METHODDEF    \
+    {"__exit__", (PyCFunction)(void(*)(void))blob_exit, METH_FASTCALL, blob_exit__doc__},
+
+static PyObject *
+blob_exit_impl(pysqlite_Blob *self, PyObject *type, PyObject *val,
+               PyObject *tb);
+
+static PyObject *
+blob_exit(pysqlite_Blob *self, PyObject *const *args, Py_ssize_t nargs)
+{
+    PyObject *return_value = NULL;
+    PyObject *type;
+    PyObject *val;
+    PyObject *tb;
+
+    if (!_PyArg_CheckPositional("__exit__", nargs, 3, 3)) {
+        goto exit;
+    }
+    type = args[0];
+    val = args[1];
+    tb = args[2];
+    return_value = blob_exit_impl(self, type, val, tb);
+
+exit:
+    return return_value;
+}
+/*[clinic end generated code: output=ca2400862c18dadb input=a9049054013a1b77]*/