Add extensions support to GraphQLError

[odero]

No description provided.

[nikordaris]

@syrusakbary can we get this pulled in? This is a big blocker on our ability to pass error code metadata to the client with graphql-python.

[nikordaris]

To add some more context for this PR. This is enabling the optional capability of the GraphQL spec that allows generic Map data to be included in the { errors: []} section of the response. The spec mandates this data be in the extensions field of an error. {errors: [{extensions: {}}]}.

https://graphql.github.io/graphql-spec/June2018/#sec-Errors

GraphQL services may provide an additional entry to errors with key extensions. This entry, if set, must have a map as its value. This entry is reserved for implementors to add additional information to errors however they see fit, and there are no additional restrictions on its contents.

My company is currently using this to pass error codes to the client as well as field level validation. Because the extensions section allows free form Map data the schema is up to us to decide what we want to include. We decided to follow this format:

{
  errors: [{
    extensions: {
      error: {
        error_code: "MyErrorCode",
        error_fields: [{path: "name", error: "Custom field error message"}],
        support_id: "1234-1234"
      }
    }
  }]
}

This PR makes defining the extensions data as a simple kwarg off of the GraphQLError type so that when I detect an error in my resolver i can simple raise GraphQLError('Foobar', extensions={'error': {'error_code': 'MyErrorCode'}}) or for convenience i can create my own Error type that will have kwargs for my specific error schema that would then get wrapped into the extensions field.

[ekampf]

This return None seems unnecessary.
Any idea why it's here?

[nikordaris]

My guess would be for the typings check. It was there prior to this PR though

[ekampf]

👍

[ekampf]

Ok approving.
Also made sure there's a feature parity with graphql-core-next: https://github.com/graphql-python/graphql-core-next/blob/72171604d160774e384f51b74afb14c7e1b01b91/graphql/error/graphql_error.py#L79