validator.rst

Validating GraphQL Queries

.. currentmodule:: graphql.validation

When executing GraphQL queries, the second step that happens under the hood after parsing the source code is a validation against the given schema using the rules of the GraphQL specification. You can also run the validation step manually by calling the :func:`validate` function, passing the schema and the AST document:

from graphql import parse, validate

errors = validate(schema, parse("""
    {
      human(id: NEWHOPE) {
        name
        homePlace
        friends
      }
    }
    """))

As a result, you will get a complete list of all errors that the validators has found. In this case, we will get the following three validation errors:

[GraphQLError(
    'String cannot represent a non string value: NEWHOPE',
    locations=[SourceLocation(line=3, column=17)]),
 GraphQLError(
    "Cannot query field 'homePlace' on type 'Human'."
     " Did you mean 'homePlanet'?",
     locations=[SourceLocation(line=5, column=9)]),
 GraphQLError(
    "Field 'friends' of type '[Character]' must have a selection of subfields."
     "  Did you mean 'friends { ... }'?",
     locations=[SourceLocation(line=6, column=9)])]

These rules are available in the :data:`specified_rules` list and implemented in the :mod:`graphql.validation.rules` subpackage. Instead of the default rules, you can also use a subset or create custom rules. The rules are based on the :class:`ValidationRule` class which is based on the :class:`~graphql.language.Visitor` class which provides a way of walking through an AST document using the visitor pattern.