Create a primer section for the descriptor howto guide #22906
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Is it possible to give more clarity on the attribute resolution method? The problem I had is that from this how-to it wasn't clear that assigning a descriptor as an attribute to a instance wouldn't make it's get and set methods resolved when that instance's attribute is called.
Co-authored-by: Eduardo Orive Vinuesa <edorka@gmail.com>
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Thanks @rhettinger for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9. |
(cherry picked from commit 8d3d731) Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>
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GH-22918 is a backport of this pull request to the 3.9 branch. |
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| Descriptors get invoked by the dot operator during attribute lookup. If a | ||
| descriptor is accessed indirectly with ``vars(some_class)[descriptor_name]``, | ||
| the descriptor instance is returned without invoking it. |
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I wonder if it is worth mentioning here the more usual convention for returning the descriptor instance (checking if obj is None)? The indirection vars(vars(Person)['name']) is a little strange compared to vars(Person.name).
| def newfunc(*args): | ||
| return self.f(klass, *args) | ||
| return self.f(cls, *args) | ||
| return newfunc |
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Instead of using newfunc, wouldn't it be better to return types.MethodType(self.f, cls) similar to what is done in Function?
| klass = type(obj) | ||
| def __get__(self, obj, cls=None): | ||
| if cls is None: | ||
| cls = type(obj) |
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The pure Python version doesn't capture the change #8405, which will call self.f.__get__(cls) if self.f has __get__. I think this behavior is surprising (and a little weird) and should be captured in this document, which doesn't shy away from nitty gritty details. I also happen to think this behavior of calling f.__get__ is incomplete and should instead call self.f.__get__(cls, cls).
Work in progress. A couple of sections remain.