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 abcc3d7 by Jelle Zijlstra in branch 'main':
bpo-46414: Add typing.reveal_type (bpo-30646)
abcc3d7