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JelleZijlstra

opened on Jan 17, 2022

Issue body actions

BPO	46414
Nosy	@gvanrossum, @srittau, @JelleZijlstra, @sobolevn, @Fidget-Spinner, @AlexWaygood, @cdce8p
PRs	bpo-46414: Add typing.reveal_type #30646

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/JelleZijlstra'
closed_at = <Date 2022-03-05.17:53:23.394>
created_at = <Date 2022-01-17.15:02:22.942>
labels = ['library', '3.11']
title = 'Add typing.reveal_type'
updated_at = <Date 2022-03-05.17:53:23.394>
user = 'https://github.com/JelleZijlstra'

bugs.python.org fields:

activity = <Date 2022-03-05.17:53:23.394>
actor = 'JelleZijlstra'
assignee = 'JelleZijlstra'
closed = True
closed_date = <Date 2022-03-05.17:53:23.394>
closer = 'JelleZijlstra'
components = ['Library (Lib)']
creation = <Date 2022-01-17.15:02:22.942>
creator = 'JelleZijlstra'
dependencies = []
files = []
hgrepos = []
issue_num = 46414
keywords = ['patch']
message_count = 2.0
messages = ['410794', '412333']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'srittau', 'JelleZijlstra', 'sobolevn', 'kj', 'AlexWaygood', 'cdce8p']
pr_nums = ['30646']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46414'
versions = ['Python 3.11']

Activity

1. JelleZijlstra commented on Jan 17, 2022

   

   JelleZijlstra

   on Jan 17, 2022

   MemberAuthor

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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-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.
2. JelleZijlstra

   added

   3.11only security fixesonly security fixes

   on Jan 17, 2022
3. JelleZijlstra

   self-assigned this

   on Jan 17, 2022
4. JelleZijlstra

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   3.11only security fixesonly security fixes

   on Jan 17, 2022
5. JelleZijlstra

   self-assigned this

   on Jan 17, 2022
6. JelleZijlstra

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Jan 17, 2022
7. gvanrossum commented on Feb 2, 2022

   

   gvanrossum

   on Feb 2, 2022

   Member

   More actions

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

   closed this as completedon Mar 5, 2022
9. JelleZijlstra

   closed this as completedon Mar 5, 2022
10. ezio-melotti

    transferred this issue fromon Apr 10, 2022

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