The `reveal_type()` primitive is injected by type checkers into the builtins. When the type checker sees a call, it prints the inferred type of the argument.

This has been implemented across all type checkers, but adding an implementation to `typing` would help document the behavior and make it more discoverable for users. Also, it means code with `reveal_type()` calls can run without runtime errors, useful if you want to run your tests at the same time as you're debugging a typing issue.

The runtime implementation can be very simple:

    def reveal_type(obj: _T, /) -> _T:
        print("Runtime type is", type(obj))
        return obj


reveal_type() is supported by all type checkers that I'm aware of (docs include https://google.github.io/pytype/faq.html#can-i-find-out-what-pytype-thinks-the-type-of-my-expression-is for pytype and https://mypy.readthedocs.io/en/stable/common_issues.html#reveal-type for mypy).

One area of divergence is the return value. Pyright returns the inferred type of the expression as a string (and uses that in its test suite for testing type inference). Mypy returns the argument, which has the advantage that you can insert `reveal_type()` in the middle of an expression without having to put it on its own line. Also, the Pyright behavior cannot sensibly be implemented at runtime. Therefore, I suggest using Mypy's behavior for `typing.reveal_type`.

New changeset abcc3d75f6e570519cb37c62130a2295c6928bff by Jelle Zijlstra in branch 'main':
bpo-46414: Add typing.reveal_type (#30646)
https://github.com/python/cpython/commit/abcc3d75f6e570519cb37c62130a2295c6928bff