Class Properties in Python
In Python, class methods can be added using the @classmethod decorator. But what about class properties? Is there a decorator for them?
Consider this code:
class Example(object): the_I = 10 def __init__(self): self.an_i = 20 @property def i(self): return self.an_i def inc_i(self): self.an_i += 1 # is this even possible? @classproperty def I(cls): return cls.the_I @classmethod def inc_I(cls): cls.the_I += 1
The code defines a class Example with an instance property i, a class property I, and two class methods inc_i and inc_I.
However, the syntax used for @classproperty isn't correct in Python. To create a class property, we can use the following approach:
class ClassPropertyDescriptor(object): def __init__(self, fget, fset=None): self.fget = fget self.fset = fset def __get__(self, obj, klass=None): if klass is None: klass = type(obj) return self.fget.__get__(obj, klass)() def __set__(self, obj, value): if not self.fset: raise AttributeError("can't set attribute") type_ = type(obj) return self.fset.__get__(obj, type_)(value) def setter(self, func): if not isinstance(func, (classmethod, staticmethod)): func = classmethod(func) self.fset = func return self def classproperty(func): if not isinstance(func, (classmethod, staticmethod)): func = classmethod(func) return ClassPropertyDescriptor(func)
With this helper code, we can define class properties as follows:
class Bar(object): _bar = 1 @classproperty def bar(cls): return cls._bar @bar.setter def bar(cls, value): cls._bar = value
The classproperty decorator creates a descriptor that handles the get and set operations for the class property.
By adding a metaclass definition, we can also handle setting the class property directly on the class:
class ClassPropertyMetaClass(type): def __setattr__(self, key, value): if key in self.__dict__: obj = self.__dict__.get(key) if obj and type(obj) is ClassPropertyDescriptor: return obj.__set__(self, value) return super(ClassPropertyMetaClass, self).__setattr__(key, value) Bar.__metaclass__ = ClassPropertyMetaClass
Now, both instance and class properties can be used and set as expected.
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