Cast error message to string to handle proxy objects

src/graphql/error/graphql_error.py

@@ -173,7 +173,7 @@ def __str__(self) -> str:
         # Lazy import to avoid a cyclic dependency between error and language
         from ..language.print_location import print_location, print_source_location
 
-        output = [self.message]
+        output = [str(self.message)]
 
         if self.nodes:
             for node in self.nodes:

tests/error/test_graphql_error.py

@@ -351,6 +351,30 @@ def prints_an_error_with_nodes_from_different_sources():
             """
         )
 
+    def handles_proxy_error_messages():
+        class ProxyString:
+            def __init__(self, value):
+                self.value = value
+
+            def __str__(self):
+                return self.value
+
+        error = GraphQLError(ProxyString("Example error"))  # type: ignore
+
+        assert str(error) == dedent(
+            """
+            Example error
+            """
+        )
+
+        error = GraphQLError(ValueError(ProxyString("Example value error")))  # type: ignore # noqa: E501
+
+        assert str(error) == dedent(
+            """
+            Example value error
+            """
+        )
+
 
 def describe_formatted():
     def deprecated_formats_an_error_using_format_error():