Add test to verify bug is fixed.

Author: Schwarf

src/duckdb_py/pyrelation.cpp

@@ -30,14 +30,16 @@ std::string QuoteIdentifier(const std::string &name) {
 	out.reserve(name.size() + 2);
 	out.push_back('"');
 	for (char c : name) {
-		if (c == '"')
+		if (c == '"') {
 			out.push_back('"'); // escape " as ""
+		}
 		out.push_back(c);
 	}
 	out.push_back('"');
 	return out;
 }
 } // namespace
+
 DuckDBPyRelation::DuckDBPyRelation(shared_ptr<Relation> rel_p) : rel(std::move(rel_p)) {
 	if (!rel) {
 		throw InternalException("DuckDBPyRelation created without a relation");

tests/fast/test_relation.py

@@ -688,3 +688,11 @@ def create_view(con, view_name: str) -> None:
 
         res = con.sql("select * from vw").fetchall()
         assert res == expected
+
+    def test_relation_select_dtypes_quotes_identifiers_with_spaces(self, duckdb_cursor):
+        # Regression test for select_dtypes on columns requiring identifier quoting (e.g., spaces)
+        df = pd.DataFrame({"na me": ["alice", "bob"], "x": [1, 2]})
+        rel = duckdb_cursor.from_df(df)
+        out = rel.select_dtypes([duckdb.sqltypes.VARCHAR]).fetchdf()
+        assert list(out.columns) == ["na me"]
+        assert out["na me"].tolist() == ["alice", "bob"]