Python 2: Forbid type objects as keys. #163
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I apologize for being unclear. I admit to mixing in the fix for #153 ("Python 3 forbids classes as keys, but Python 2 doesn't") with some semi-unrelated cleanup. The direct fix for #153 was to specifically check It turns out there are a lot of differences among the supported implementations of Python as to how "doesn't implement comparison" can be detected. The refactoring changes both simplified and improved the speed of that. (I hope.) Please let me know if I can help clarify further. I'd very much like to get this merged and released. |
Fixes #153. Also take the opportunity to clean up the TODOs in _datatypes.py with regards to checking for default comparison: Use a metaclass so we get caching, and only check for the special methods on the type object.
| def __call__(self, item): | ||
| self._check_default_comparison(item) | ||
| if isinstance(item, _HasDefaultComparison): |
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_HasDefaultComparison is defined as a new class above but where is item registered for it, so this check can become true?
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_HasDefaultComparison is defined as a new class above
Right. Specifically, it's defined as a new abstract base class, i.e., something that extends abc.ABCMeta. (We could do this with a regular type subclass, but extending ABCMeta gets us caching of the results of isinstance automatically.)
where is
itemregistered for it, so this check can become true?
That's the beauty part (as the saying goes). By making _HasDefaultComparison implement __subclasshook__, we algorithmically define what isinstance(item, _HasDefaultComparison) means. No need to register anything! The algorithm is the same as what was here before, just moved into __subclasshook__. (The expression isinstance(item, Cls) basically boils down to Cls.__subclasscheck__(type(item)); ABCMeta defines __subclasscheck__ to do some caching, and if the answer is not in the cache, to call Cls.__subclasshook__().)
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Did that help clear things up sufficiently?
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TIL, I think I now understood how it works. Maybe adding a link to the Python documentation would help future readers of the code who rarely use abstract base classes.
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Thank you! |
Fixes #153.
Also take the opportunity to clean up the TODOs in _datatypes.py with regards to checking for default comparison: Use a metaclass so we get caching, and only check for the special methods on the type object.