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POpython attribute lookup without any descriptor magic?
 

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  1. POpython attribute lookup without any descriptor magic?
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    <p>I've started to use the python descriptor protocol more extensively in the code I've been writing. Typically, the default python lookup magic is what I want to happen, but sometimes I'm finding I want to get the descriptor object itself instead the results of its <code>__get__</code> method. Wanting to know the type of the descriptor, or access state stored in the descriptor, or somesuch thing.</p> <p>I wrote the code below to walk the namespaces in what I believe is the correct ordering, and return the attribute raw regardless of whether it is a descriptor or not. I'm surprised though that I can't find a built-in function or something in the standard library to do this -- I figure it has to be there and I just haven't noticed it or googled for the right search term.</p> <p>Is there functionality somewhere in the python distribution that already does this (or something similar)?</p> <p>Thanks!</p> <pre><code>from inspect import isdatadescriptor def namespaces(obj): obj_dict = None if hasattr(obj, '__dict__'): obj_dict = object.__getattribute__(obj, '__dict__') obj_class = type(obj) return obj_dict, [t.__dict__ for t in obj_class.__mro__] def getattr_raw(obj, name): # get an attribute in the same resolution order one would normally, # but do not call __get__ on the attribute even if it has one obj_dict, class_dicts = namespaces(obj) # look for a data descriptor in class hierarchy; it takes priority over # the obj's dict if it exists for d in class_dicts: if name in d and isdatadescriptor(d[name]): return d[name] # look for the attribute in the object's dictionary if obj_dict and name in obj_dict: return obj_dict[name] # look for the attribute anywhere in the class hierarchy for d in class_dicts: if name in d: return d[name] raise AttributeError </code></pre> <p>Edit Wed, Oct 28, 2009.</p> <p>Denis's answer gave me a convention to use in my descriptor classes to get the descriptor objects themselves. But, I had an entire class hierarchy of descriptor classes, and I didn't want to begin <em>every</em> <code>__get__</code> function with a boilerplate</p> <pre><code>def __get__(self, instance, instance_type): if instance is None: return self ... </code></pre> <p>To avoid this, I made the root of the descriptor class tree inherit from the following:</p> <pre><code>def decorate_get(original_get): def decorated_get(self, instance, instance_type): if instance is None: return self return original_get(self, instance, instance_type) return decorated_get class InstanceOnlyDescriptor(object): """All __get__ functions are automatically wrapped with a decorator which causes them to only be applied to instances. If __get__ is called on a class, the decorator returns the descriptor itself, and the decorated __get__ is not called. """ class __metaclass__(type): def __new__(cls, name, bases, attrs): if '__get__' in attrs: attrs['__get__'] = decorate_get(attrs['__get__']) return type.__new__(cls, name, bases, attrs) </code></pre>
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    1. USMatt Anderson
    1. USMatt Anderson
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    1. COSometimes you want the descriptor object? That violates the core expectation for descriptors: they're supposed to look like attributes. Why break that fundamental expectation? Why do this? Why create something so complex?
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      1. USS.Lott
    2. COWhat I'm doing doesn't feel *that* complex to me, but I guess you could say I'm experimenting with the design. In my current, particular case, I have a descriptor that returns the strength of a weapon in a game. That value is a function of the descriptor's state (strength of the weapon) and the instance (health of the ship). There are different kinds of weapon; usually I just want the value result, but in a few cases, I need to know what kind of weapon it is -- the type of the descriptor. And what if a descriptor has methods not part of the descriptor protocol, and you want to call them?
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      1. USMatt Anderson
 

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