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# from jaraco.classes 3.2.2
class NonDataProperty:
"""Much like the property builtin, but only implements __get__,
making it a non-data property, and can be subsequently reset.
See http://users.rcn.com/python/download/Descriptor.htm for more
information.
>>> class X(object):
... @NonDataProperty
... def foo(self):
... return 3
>>> x = X()
>>> x.foo
3
>>> x.foo = 4
>>> x.foo
4
"""
def __init__(self, fget):
assert fget is not None, "fget cannot be none"
assert callable(fget), "fget must be callable"
self.fget = fget
def __get__(self, obj, objtype=None):
if obj is None:
return self
return self.fget(obj)
class classproperty:
"""
Like @property but applies at the class level.
>>> class X(metaclass=classproperty.Meta):
... val = None
... @classproperty
... def foo(cls):
... return cls.val
... @foo.setter
... def foo(cls, val):
... cls.val = val
>>> X.foo
>>> X.foo = 3
>>> X.foo
3
>>> x = X()
>>> x.foo
3
>>> X.foo = 4
>>> x.foo
4
Setting the property on an instance affects the class.
>>> x.foo = 5
>>> x.foo
5
>>> X.foo
5
>>> vars(x)
{}
>>> X().foo
5
Attempting to set an attribute where no setter was defined
results in an AttributeError:
>>> class GetOnly(metaclass=classproperty.Meta):
... @classproperty
... def foo(cls):
... return 'bar'
>>> GetOnly.foo = 3
Traceback (most recent call last):
...
AttributeError: can't set attribute
It is also possible to wrap a classmethod or staticmethod in
a classproperty.
>>> class Static(metaclass=classproperty.Meta):
... @classproperty
... @classmethod
... def foo(cls):
... return 'foo'
... @classproperty
... @staticmethod
... def bar():
... return 'bar'
>>> Static.foo
'foo'
>>> Static.bar
'bar'
*Legacy*
For compatibility, if the metaclass isn't specified, the
legacy behavior will be invoked.
>>> class X:
... val = None
... @classproperty
... def foo(cls):
... return cls.val
... @foo.setter
... def foo(cls, val):
... cls.val = val
>>> X.foo
>>> X.foo = 3
>>> X.foo
3
>>> x = X()
>>> x.foo
3
>>> X.foo = 4
>>> x.foo
4
Note, because the metaclass was not specified, setting
a value on an instance does not have the intended effect.
>>> x.foo = 5
>>> x.foo
5
>>> X.foo # should be 5
4
>>> vars(x) # should be empty
{'foo': 5}
>>> X().foo # should be 5
4
"""
class Meta(type):
def __setattr__(self, key, value):
obj = self.__dict__.get(key, None)
if type(obj) is classproperty:
return obj.__set__(self, value)
return super().__setattr__(key, value)
def __init__(self, fget, fset=None):
self.fget = self._ensure_method(fget)
self.fset = fset
fset and self.setter(fset)
def __get__(self, instance, owner=None):
return self.fget.__get__(None, owner)()
def __set__(self, owner, value):
if not self.fset:
raise AttributeError("can't set attribute")
if type(owner) is not classproperty.Meta:
owner = type(owner)
return self.fset.__get__(None, owner)(value)
def setter(self, fset):
self.fset = self._ensure_method(fset)
return self
@classmethod
def _ensure_method(cls, fn):
"""
Ensure fn is a classmethod or staticmethod.
"""
needs_method = not isinstance(fn, (classmethod, staticmethod))
return classmethod(fn) if needs_method else fn
|
