Type hints annotate Python values, parameters, and returns with expected types. Python still uses duck typing and does not enforce these hints at runtime, but they document intent, make unfamiliar code easier to maintain, and support static checking. section 2.7.2
Clarify a balance-formatting function
A balance formatter can declare its input as str and its return as str: def print_balance(current_balance: str) -> str. Returning the formatted value rather than printing it lets a caller print it, write it to a file, or use it in an API response. section 1.5.2
Hints are especially useful when a dictionary selects parser functions. Without them, it can be unclear whether dictionary values are callable or which arguments they accept. A Callable annotation records that each selected parser receives one string and returns a dictionary of strings. section 2.7.1 section 2.7.2
parser_functions: Dict[str, Callable[[str], Dict[str, str]]] = {
This annotation compactly describes both sides of a dispatch table: string record-type keys and callable parser values with a declared input and result. section 2.7.2
Use hints to make an evolving application safer to navigate:
- Annotate function parameters and return values so callers can see the intended contract. section 1.5.2
- Add static checking with mypy, Pyright, or Pyre when a project needs stronger feedback than runtime duck typing provides. section 2.7.2
