fix: Internal error on some invalid schemas

Author: Stranger6667

docs/changelog.rst

@@ -4,6 +4,10 @@ Changelog
 `Unreleased`_ - TBD
 -------------------
 
+**Fixed**
+
+- Internal error on some invalid schemas.
+
 `0.8.1`_ - 2022-04-27
 ---------------------
 

src/hypothesis_graphql/_strategies/strategy.py

@@ -317,6 +317,10 @@ def subset_of_fields(
     fields: Dict[str, graphql.GraphQLInputField], *, force_required: bool = False
 ) -> st.SearchStrategy[List[Tuple[str, graphql.GraphQLInputField]]]:
     """A helper to select a subset of fields."""
+    if not fields:
+        # The schema is invalid as there should be at least one field
+        # But there should not be an internal error because of it
+        return EMPTY_LISTS_STRATEGY
     field_pairs = sorted(fields.items())
     # if we need to always generate required fields, then return them and extend with a subset of optional fields
     if force_required:

test/test_queries.py

@@ -129,9 +129,8 @@ def test_arguments(data, schema, arguments, node_names, notnull, validate_operat
 @given(data=st.data())
 def test_interface(data, schema, query_type, validate_operation):
     schema = schema + query_type
-    parsed_schema = cached_build_schema(schema)
     query = data.draw(gql_st.queries(schema))
-    validate_operation(parsed_schema, query)
+    validate_operation(schema, query)
 
 
 @pytest.mark.parametrize(
@@ -453,3 +452,26 @@ def validate_and_find(query):
 
     find(strategy, validate_and_find)
     assert all_valid
+
+
+@given(data=st.data())
+def test_empty_interface(data, validate_operation):
+    # When the schema contains an empty interface (which is invalid)
+    schema = """
+interface Empty
+
+type First implements Empty {
+  int: Int!
+}
+type Second implements Empty {
+  float: Float!
+}
+
+type Query {
+  getByEmpty: Empty
+}"""
+    # Then query generation should not fail
+    query = data.draw(gql_st.queries(schema))
+    # And then schema validation should fail instead
+    with pytest.raises(TypeError, match="Type Empty must define one or more fields"):
+        validate_operation(schema, query)