NoReturn vs. None in "void" functions - type annotations in Python 3.6
NoReturn means the function never returns a value.
The function either does not terminate or always throws an exception: "The typing module provides a special type NoReturn to annotate functions that never return normally. For example, a function that unconditionally raises an exception..".
from typing import NoReturndef stop() -> NoReturn: raise RuntimeError('no way')That is, x = foo_None() is type-valid but suspect while x = foo_NoReturn() is invalid.
Besides never having an assignable result, NoReturn also has other implications in branch analysis: foo_NoReturn(); unreachable... There is further discussion in the 'A NoReturn type is needed #165' ticket.
In order to perform branch analysis, it is necessary to know which calls will never return normally. Examples are
sys.exit(which always returns via exception) andos.exit(which never returns)..