New submission from Jelle Zijlstra <[email protected]>:
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`.
----------
assignee: Jelle Zijlstra
components: Library (Lib)
messages: 410794
nosy: AlexWaygood, Jelle Zijlstra, gvanrossum, kj
priority: normal
severity: normal
status: open
title: Add typing.reveal_type
versions: Python 3.11
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Python tracker <[email protected]>
<https://bugs.python.org/issue46414>
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