Validating GraphQL Queries

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 validate() function, passing the schema and the AST document:

from graphql import parse, validate

errors = validate(schema, parse("""
      human(id: NEWHOPE) {

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:

    'String cannot represent a non string value: NEWHOPE',
    locations=[SourceLocation(line=3, column=17)]),
    "Cannot query field 'homePlace' on type 'Human'."
     " Did you mean 'homePlanet'?",
     locations=[SourceLocation(line=5, column=9)]),
    "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 specified_rules list and implemented in the 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 ValidationRule class which is based on the Visitor class which provides a way of walking through an AST document using the visitor pattern.