Application formatting error

If you’ve experimented with GraphQL, you should be familiar that when things don’t go according to plan, GraphQL servers include an additional key errors in the returned response:

{
    "errors": [
        {
            "message": "Variable "$input" got invalid value {}.nIn field "name": Expected "String!", found null.",
            "locations": [
                {
                    "line": 1,
                    "column": 21
                }
            ]
        }
    ]
}

Your first instinct when planning error messaging may be to use this approach to communicate custom errors (like permission or validation errors) raised by your resolvers.

Don’t do this.

The errors key is, by design, supposed to relay errors to other developers working with the API. Messages present under this key are technical in nature and shouldn’t be displayed to your end users.

Instead, you should define custom fields that your queries and mutations will include in result sets, to relay eventual errors and problems to clients, like this:

type_def = """
    type Mutation {
        login(username: String!, password: String!) {
            error: String
            user: User
        }
    }
"""

Depending on success or failure, your mutation resolver may return either an error message to be displayed to the user, or user that has been logged in. Your API result-handling logic may then interpret the response based on the content of those two keys, only falling back to the main errors key to make sure there wasn’t an error in query syntax, connection or application.

Likewise, your Query resolvers may return None instead of requested object, which client developers may interpret as a signal from API to display «Requested item doesn’t exist» message the user in place of the requested resource.

Debugging errors

By default individual errors elements contain a very limited amount of information about errors occurring inside the resolvers, forcing a developer to search an application’s logs for details about the error’s possible causes.

Developer experience can be improved by including debug=True in the list of arguments passed to Ariadne’s GraphQL object:

app = GraphQL(schema, debug=True)

This will result in each error having an additional exception key containing both a complete stacktrace and current context for which the error has occurred:

{
    "errors": [
        {
            "message": "'dict' object has no attribute 'build_name'",
            "locations": [
                [
                    3,
                    5
                ]
            ],
            "path": [
                "people",
                0,
                "fullName"
            ],
            "extensions": {
                "exception": {
                    "stacktrace": [
                        "Traceback (most recent call last):",
                        "  File "/Users/lib/python3.6/site-packages/graphql/execution/execute.py", line 619, in resolve_field_value_or_error",
                        "    result = resolve_fn(source, info, **args)",
                        "  File "myapp.py", line 40, in resolve_person_fullname",
                        "    return get_person_fullname(person)",
                        "  File "myapp.py", line 47, in get_person_fullname",
                        "    return person.build_name()",
                        "AttributeError: 'dict' object has no attribute 'build_name'"
                    ],
                    "context": {
                        "person": "{'firstName': 'John', 'lastName': 'Doe', 'age': 21}"
                    }
                }
            }
        }
    ]
}

Replacing default error formatter

The default error formatter used by Ariadne performs the following tasks:

• Formats error by using its formatted property.
• Recursively unwraps the GraphQL error by accessing its original_error property.
• If the unwrapped error is available and the debug argument is set to True, update the already formatted error to also include an extensions entry with an exception dictionary containing stacktrace and context.

If you wish to change or customize this behavior, you can define a custom function in the error_formatter argument to a GraphQL object:

from ariadne import format_error

def my_format_error(error: GraphQLError, debug: bool = False) -> dict:
    if debug:
        # If debug is enabled, reuse Ariadne's formatting logic (not required)
        return format_error(error, debug)

    # Create formatted error data
    formatted = error.formatted
    # Replace original error message with custom one
    formatted["message"] = "INTERNAL SERVER ERROR"
    return formatted

app = GraphQL(schema, error_formatter=my_format_error)