Add role parameter

Author: damian3031

README.md

@@ -359,6 +359,18 @@ conn = trino.dbapi.connect(
 )
 ```
 
+You could also pass `system` role without explicitly specifing "system" catalog:
+
+```python
+import trino
+conn = trino.dbapi.connect(
+    host='localhost',
+    port=443,
+    user='the-user',
+    roles="roleC" # equivalent to {"system": "roleC"} 
+)
+```
+
 ## Timezone
 
 The time zone for the session can be explicitly set using the IANA time zone

tests/integration/test_dbapi_integration.py

@@ -1144,6 +1144,18 @@ def test_set_role_in_connection(run_trino):
     assert_role_headers(cur, "system=ALL")
 
 
+def test_set_system_role_in_connection(run_trino):
+    _, host, port = run_trino
+
+    trino_connection = trino.dbapi.Connection(
+        host=host, port=port, user="test", catalog="tpch", roles="ALL"
+    )
+    cur = trino_connection.cursor()
+    cur.execute('SHOW TABLES FROM information_schema')
+    cur.fetchall()
+    assert_role_headers(cur, "system=ALL")
+
+
 def assert_role_headers(cursor, expected_header):
     assert cursor._request.http_headers[constants.HEADER_ROLE] == expected_header
 

trino/client.py

@@ -135,7 +135,7 @@ def __init__(
         transaction_id: str = None,
         extra_credential: List[Tuple[str, str]] = None,
         client_tags: List[str] = None,
-        roles: Dict[str, str] = None,
+        roles: Union[Dict[str, str], str] = None,
         timezone: str = None,
     ):
         self._user = user
@@ -239,6 +239,8 @@ def timezone(self):
             return self._timezone
 
     def _format_roles(self, roles):
+        if isinstance(roles, str):
+            roles = {"system": roles}
         formatted_roles = {}
         for catalog, role in roles.items():
             is_legacy_role_pattern = ROLE_PATTERN.match(role) is not None