Step 3: Parameters and Type Checking

JSON-RPC supports both positional and keyword parameters. Flask-JSONRPC parses parameters automatically and supports validation using Python type hints.

In this step, you will:

• use positional parameters

• use keyword parameters

• learn how type hints enable validation

---

Positional Parameters

Here is a simple method that expects two integers:

@jsonrpc.method("math.sub")
def subtract(a: int, b: int) -> int:
    return a - b

Call it using positional parameters:

curl -X POST http://127.0.0.1:5000/api \
     -H "Content-Type: application/json" \
     -d '{"jsonrpc":"2.0","method":"math.sub","params":[10,4],"id":1}'

---

Keyword Parameters

JSON-RPC also supports keyword arguments:

@jsonrpc.method("greeting.hello")
def hello(name: str) -> str:
    return f"Hello, {name}!"

Call it with:

{
  "jsonrpc": "2.0",
  "method": "greeting.hello",
  "params": {"name": "Alice"},
  "id": 2
}

---

Type Validation

Because you annotated your method like:

def subtract(a: int, b: int) -> int:

Flask-JSONRPC will perform basic type validation.

Try sending a string:

curl -X POST http://127.0.0.1:5000/api \
     -H "Content-Type: application/json" \
     -d '{"jsonrpc":"2.0","method":"math.sub","params":["foo",4],"id":1}'

You will receive a JSON-RPC error indicating that a was expected to be an integer.

Later, in the Patterns section, you will learn to extend this system using Pydantic for complex validation.