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Allow NewType subclassing NewType. - #3465
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Use case: type hierarchy of IDs, all of which are integers at runtime, but we want to type-check distinctions between e.g. "id of User" and "id of Media", while still allowing some functions to take "any ID" (which is still a more specific type than "any integer").
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Yeah, PEP 484 says that the second arg to |
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(And yes, it works fine at runtime.) |
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I like the idea. This will require a small update to PEP 484. This indeed works at runtime (even for runtime introspection tools). For example: Id = NewType('Id', int)
UserId = NewType('UserId', Id)
assert UserId.__supertype__.__name__ == 'Id'
assert UserId.__supertype__.__supertype__ is int |
ilevkivskyi
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May 28, 2017
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OK, it does make sense, and go ahead with the PEP 484 update. |
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gvanrossum
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May 30, 2017
See discussion in python/mypy#3465
carljm
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* master: (23 commits) Make return type of open() more precise (python#3477) Add test cases that delete a file during incremental checking (python#3461) Parse each format-string component separately (python#3390) Don't warn about returning Any if it is a proper subtype of the return type (python#3473) Add __setattr__ support (python#3451) Remove bundled lib-typing (python#3337) Move version of extensions to post-release (python#3348) Fix None slice bounds with strict-optional (python#3445) Allow NewType subclassing NewType. (python#3465) Add console scripts (python#3074) Fix 'variance' label. Change label for variance section to just 'variance' (python#3429) Better error message for invalid package names passed to mypy (python#3447) Fix last character cut in html-report if file does not end with newline (python#3466) Print pytest output as it happens (python#3463) Add mypy roadmap (python#3460) Add flag to avoid interpreting arguments with a default of None as Optional (python#3248) Add type checking plugin support for functions (python#3299) Mismatch of inferred type and return type note (python#3428) Sync typeshed (python#3449) ...
topper-123
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Sep 22, 2017
It has become possible to create a ``NewType`` from an existing ``NewType``, see python/mypy#3465 and python/peps#271. This is a small update to the documentation as a concequence of the above change.
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Use case: type hierarchy of IDs, all of which are integers at runtime, but we
want to type-check distinctions between e.g. "id of User" and "id of Media",
while still allowing some functions to take "any ID" (which is still a more
specific type than "any integer").
This seems to work as expected, it only requires removing an explicit guard
against it. Is there a reason to prohibit it? I didn't find any specific rationale
in the issue (#1284) or commit. @JukkaL raised it as a question in
#1284 (comment)
but it wasn't ever discussed further.