[New-bugs-announce] [issue46414] Add typing.reveal_type

Jelle Zijlstra report at bugs.python.org
Mon Jan 17 10:02:22 EST 2022

New submission from Jelle Zijlstra <jelle.zijlstra at gmail.com>:

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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