214 – reveal_type

The module typing provides a debugging aid, the function reveal_type. When a static type checker encounters it, the static type checker emits a note with the inferred type of the expression passed into reveal_type:

def s() -> str:
    return 3

reveal_type(s())
# Revealed type is "builtins.str"

The function reveal_type doesn’t even need to be imported and the static type checker infers that s() is a string because of the return type -> str. (The static type checker also complains about the fact that we’re returning an integer from inside s while the function should return a string, but you’ll see why I’m doing that in a second.)

At runtime, the function reveal_type prints the runtime type of the argument to sys.stderr and returns the argument unchanged:

from typing import reveal_type

def s() -> str:
    return 3

reveal_type(s())
# Runtime type is 'int'

At runtime, the type of s() is int because the function s returns the integer 3. For the code to work at runtime, typing.reveal_type must be imported since it’s not a built-in.