Bug in databricks.sql.exc.Error base class (#437)

src/databricks/sql/exc.py

@@ -37,10 +37,10 @@ def __init__(
             )
 
     def __str__(self):
-        return self.message
+        return self.message or ""
 
     def message_with_context(self):
-        return self.message + ": " + json.dumps(self.context, default=str)
+        return (self.message or "") + ": " + json.dumps(self.context, default=str)
 
 
 class Warning(Exception):

tests/unit/test_exc.py

@@ -0,0 +1,19 @@
+import databricks.sql.exc as exc
+
+
+class TestError:
+    def test_str_returns_string_when_message_is_none(self):
+        """object.__str__ must return a str; a None message must not raise
+        TypeError: __str__ returned non-string (type NoneType)."""
+        e = exc.Error()
+        result = str(e)
+        assert isinstance(result, str)
+
+    def test_message_with_context_when_message_is_none(self):
+        """message_with_context must not raise on `None + str` when message
+        is None; it should still serialize the context."""
+        e = exc.Error(context={"foo": "bar"})
+        result = e.message_with_context()
+        assert isinstance(result, str)
+        assert "foo" in result
+        assert "bar" in result