As a static type checking amateur, it's not clear to me what the aim of adding this to stdlib is. Is this to avoid the pitfalls of code containing calls to `reveal_type` when not running mypy or other static type checker? Is the intent for the static type checker to monkey-patch the typing library to provide something different when it's running? On Mon, 2022-01-17 at 07:14 -0800, Jelle Zijlstra wrote:
I just opened https://bugs.python.org/issue46414 to propose adding `typing.reveal_type` to Python 3.11. Copying my message there:
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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-t... 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`.
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I'm hoping this will be simple and noncontroversial enough that we can add it quickly, but if there are concerns from any type checker authors, I'd love to hear them. _______________________________________________ Typing-sig mailing list -- typing-sig@python.org To unsubscribe send an email to typing-sig-leave@python.org https://mail.python.org/mailman3/lists/typing-sig.python.org/ Member address: pbryan@anode.ca