gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sentinels#149086
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JelleZijlstra merged 1 commit intopython:mainfrom Apr 30, 2026
Merged
gh-149083: Convert dataclasses.MISSING and dataclasses.KW_ONLY to sentinels#149086JelleZijlstra merged 1 commit intopython:mainfrom
JelleZijlstra merged 1 commit intopython:mainfrom
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…to sentinels There were comments claiming these were implemented as custom classes to give a nicer repr(), but the repr() wasn't all that nice: >>> repr(dataclasses.MISSING) '<dataclasses._MISSING_TYPE object at 0x1005e7e00>' >>> repr(dataclasses.KW_ONLY) '<dataclasses._KW_ONLY_TYPE object at 0x100884050>' Sentinels are conceptually the right tool for these, so let's use them. This does change the repr() of these two objects.
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Thanks, @JelleZijlstra. My only concern: is there any chance this will cause a performance regression when importing dataclasses? I assume not, since there are no imports involved, but I thought I'd ask. People are understandably touchy about it. |
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It's almost certainly faster. Creating a class is quite slow; creating a sentinel is a very simple C call. From So this might actually measurably improve import time. |
ericvsmith
approved these changes
Apr 30, 2026
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| def test_missing_repr(self): | ||
| self.assertIn('MISSING_TYPE object', repr(MISSING)) | ||
| self.assertEqual(repr(MISSING), 'MISSING') |
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I have no idea why the arguments were in the opposite order before, but changing them is a definite improvement!
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And yes, I realize that wasn't really the point of this change.
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There were comments claiming these were implemented as custom classes to give a nicer
repr(), but the repr() wasn't all that nice:
Sentinels are conceptually the right tool for these, so let's use them.
This does change the repr() of these two objects.