Python Functions

Python Functions: Parameters, Type Annotations, & Scope

Functions are reusable blocks of code defined using the def keyword. Modern Python emphasizes type hints, explicit parameter binding rules, and clean return values.

Function Parameter Syntax Signature

flowchart TD
    A["def func(pos_only, /, standard, *, kw_only):"]
    A --> B["/ : Forces Positional-Only Parameters"]
    A --> C["Standard: Positional or Keyword"]
    A --> D["* : Forces Keyword-Only Parameters"]

Parameter Types Overview

1. Positional Arguments: Bound by order of passing.
2. Keyword Arguments: Bound by explicit parameter name (func(key=val)).
3. *args: Collects excess positional arguments as a tuple.
4. **kwargs: Collects excess keyword arguments as a dictionary.

Practical Code Example

from typing import List, Dict, Any, Optional

def build_api_response(
    status_code: int,
    message: str,
    /,  # Positional-only parameters before '/'
    *,  # Keyword-only parameters after '*'
    data: Optional[Dict[str, Any]] = None,
    tags: Optional[List[str]] = None
) -> Dict[str, Any]:
    """Construct a standardized JSON API response object."""
    payload: Dict[str, Any] = {
        "status": status_code,
        "message": message,
        "success": 200 <= status_code < 300,
        "data": data or {},
        "tags": tags or []
    }
    return payload

if __name__ == "__main__":
    # Correct invocation (positional before '/', keyword after '*')
    response = build_api_response(
        200, "OK", 
        data={"user_id": 42}, 
        tags=["auth", "v1"]
    )
    print("API Response:", response)

Best Practices & Gotchas

• Never Use Mutable Defaults: def func(lst=[]) retains state across calls! Use def func(lst: list | None = None): if lst is None: lst = [].
• Return Explicit Types: Annotate return values with -> ReturnType (or -> None if no value is returned).
• Single Responsibility Principle: Each function should perform one specific, well-defined operation.

Self-Check Challenge

Write a function calculate_total(price: float, tax_rate: float = 0.05, discount: float = 0.0) -> float with complete type annotations.