import inspect
#class metamethod:
class classonlymethod:
"""Like a classmethod but does not show up on instances.
This is an alternative to putting the methods on a metaclass. It
is especially meaningful for alternate constructors.
"""
def __init__(self, method):
self.method = method
self.descr = classmethod(method)
def __get__(self, obj, cls):
name = self.method.__name__
getattr_static = inspect.getattr_static
if obj is not None:
# look up the attribute, but skip cls
dummy = type(cls.__name__, cls.__bases__, {})
attr = getattr_static(dummy(), name, None)
getter = getattr_static(attr, '__get__', None)
# try data descriptors
if (getter and getattr_static(attr, '__set__', False)):
return getter(attr, obj, cls)
# try the instance
try:
instance_dict = object.__getattribute__(obj, "__dict__")
except AttributeError:
pass
else:
try:
return dict.__getitem__(instance_dict, name)
except KeyError:
pass
# try non-data descriptors
if getter is not None:
return getter(attr, obj, cls)
raise AttributeError(name)
else:
descr = vars(self)['descr']
return descr.__get__(obj, cls)
#################################################
# tests
import unittest
class ClassOnlyMethodTests(unittest.TestCase):
def test_class(self):
class Spam:
@classonlymethod
def from_nothing(cls):
return Spam()
spam = Spam.from_nothing()
self.assertIsInstance(spam, Spam)
def test_instance(self):
class Spam:
@classonlymethod
def from_nothing(cls):
return Spam()
with self.assertRaises(AttributeError):
Spam().from_nothing()
if __name__ == '__main__':
unittest.main()
