194 – Type-safe dictionaries

You can add type safety to dictionaries with a fixed structure by using typing.TypedDict, which you define by specifying the keys and the types of the values:

from typing import TypedDict

class Config(TypedDict):
    debug_mode: bool

The typed dictionary Config is a dictionary that has the key "debug_mode", and that key maps to a Boolean value. Then, if you use the type Config in your code, your static type checker will ensure you’re using a dictionary that contains the required keys:

def run(..., c: Config) -> None:
    if c["debug_mode"]:
        print("run starting")
    ...

The key access c["debug_mode"] type checks because the dictionary c is a Config dictionary, which has the key "debug_mode" that maps to a Boolean value.

If you didn’t use a typed dictionary, you would probably say that c is a dictionary that maps strings to Booleans:

def run(..., c: dict[str, bool]) -> None:
    if c["debug_mode"]:
        print("run starting")
    ...

In this case, there is an infinite number of dictionaries that would pass type checking if passed to the function run and that would raise a KeyError because they don’t have the correct key.