Restore previous behaviour when con is a str without sqlalchemy installed

Author: abowden1989

pandas/io/sql.py

@@ -738,12 +738,14 @@ def pandasSQL_builder(con, schema: str | None = None):
 
     sqlalchemy = import_optional_dependency("sqlalchemy", errors="ignore")
 
-    if sqlalchemy is not None:
-        if isinstance(con, str):
+    if isinstance(con, str):
+        if sqlalchemy is None:
+            raise ImportError("Using URI string without sqlalchemy installed.")
+        else:
             con = sqlalchemy.create_engine(con)
 
-        if isinstance(con, sqlalchemy.engine.Connectable):
-            return SQLDatabase(con, schema=schema)
+    if sqlalchemy is not None and isinstance(con, sqlalchemy.engine.Connectable):
+        return SQLDatabase(con, schema=schema)
 
     warnings.warn(
         "pandas only support SQLAlchemy connectable(engine/connection) or"

pandas/tests/io/test_sql.py

@@ -1524,7 +1524,8 @@ def test_sql_open_close(self, test_frame3):
     @pytest.mark.skipif(SQLALCHEMY_INSTALLED, reason="SQLAlchemy is installed")
     def test_con_string_import_error(self):
         conn = "mysql://root@localhost/pandas"
-        with pytest.raises(ImportError, match="SQLAlchemy"):
+        msg = "Using URI string without sqlalchemy installed"
+        with pytest.raises(ImportError, match=msg):
             sql.read_sql("SELECT * FROM iris", conn)
 
     @pytest.mark.skipif(SQLALCHEMY_INSTALLED, reason="SQLAlchemy is installed")