Implementing a Basic GraphQL Resolver in Python

This challenge focuses on building a fundamental GraphQL resolver function in Python. Resolvers are the heart of a GraphQL API, responsible for fetching the data that corresponds to a specific field in a GraphQL query. Mastering resolvers is crucial for developing any GraphQL service in Python.

Problem Description

Your task is to implement a Python function that acts as a GraphQL resolver. This resolver will be responsible for fetching data for a hypothetical user field within a GraphQL schema. The resolver should accept obj, info, and any arguments passed in the GraphQL query and return the appropriate data.

Key Requirements:

1. Function Signature: The resolver function must adhere to a standard GraphQL resolver signature: resolver_function(obj, info, **kwargs).
   • obj: The parent object, if this resolver is part of a nested query. For the top-level user field, this will typically be None.
   • info: An object containing information about the execution state of the query.
   • **kwargs: A dictionary containing any arguments passed to the field in the GraphQL query.
2. Data Fetching Logic: The resolver should simulate fetching user data based on an id argument provided in the query.
3. Return Value: The resolver should return a dictionary representing the user's data or None if the user is not found.

Expected Behavior:

• If an id argument is provided and a corresponding user is found, return a dictionary containing the user's id and username.
• If no id argument is provided, or if the provided id does not match any existing user, return None.

Edge Cases to Consider:

• Handling missing id arguments.
• Handling invalid or non-existent id values.

Examples

Example 1:

GraphQL Query:
query {
  user(id: "1") {
    id
    username
  }
}

Simulated Resolver Call:
resolver_function(None, mock_info, id="1")

Expected Output:
{"id": "1", "username": "alice"}

Example 2:

GraphQL Query:
query {
  user(id: "99") {
    id
    username
  }
}

Simulated Resolver Call:
resolver_function(None, mock_info, id="99")

Expected Output:
None

Example 3:

GraphQL Query:
query {
  user {
    id
    username
  }
}

Simulated Resolver Call:
resolver_function(None, mock_info)

Expected Output:
None

Constraints

• The id argument will always be a string.
• The username will always be a string.
• The simulated user data will be a fixed dictionary: {"1": {"id": "1", "username": "alice"}, "2": {"id": "2", "username": "bob"}}.
• The resolver function should be efficient and execute within reasonable time limits for typical use cases.

Notes

• You will need to create a mock info object for testing purposes, as it's part of the standard resolver signature but not directly relevant to the core logic of this specific problem.
• The focus is on the logic within the resolver function itself, not on setting up a full GraphQL server or schema.
• Consider how you would handle errors or more complex data fetching scenarios in a real-world application.