Types

Literal and Final

Literal restricts exact values, while Final marks names that should not be rebound.

Literal describes exact allowed values. A type checker can reject "debug" as a Mode even though it is an ordinary string at runtime.

Source

from typing import Final, Literal

Mode = Literal["read", "write"]


def open_label(mode: Mode) -> str:
    return f"opening for {mode}"

print(open_label("write"))

Output

opening for write

Final marks a name that should not be rebound. It is stronger documentation than the all-caps constant convention because static tools can flag reassignment.

Source

DEFAULT_MODE: Final[Mode] = "read"
print(open_label(DEFAULT_MODE))

Output

opening for read

The annotation is not a runtime lock. Python still rebinds the name; the mistake is that a type checker and human reader should reject the design.

Source

DEFAULT_MODE = "debug"
print(DEFAULT_MODE)

Output

debug

Notes

Literal narrows values to a small exact set.
Final prevents rebinding in static analysis, not at runtime.
Use enums when the option set needs names, behavior, or iteration over members.

Run the complete example

Expected output

opening for read
opening for write
debug

Execution time appears here after you run the example.

Literal and Final

See also

Run the complete example

Example code

Expected output