John Machin wrote (of Alex Martelli's sketch):
Thank you for the motivation. I now understand what it is attempting to
do. Unfortunately it doesn't succeed. Instead of:
if v is notimplemented: abstract_methods.append(n)
you need:
if v.im_func is notimplemented: abstract_methods.append(n)
Here is an implementation from that sketch:
from inspect import getmembers
class MustImplement(type):
'''A metaclass to detect instantiation of abstract classes'''
def __init__(class_, name, bases, defined):
super(MustImplement, class_).__init__(name, bases, defined)
class_._abstract_methods = sorted(name
for name, value in getmembers(class_)
if value is NotImplemented)
def __call__(class_, *args, **kwargs):
if class_._abstract_methods:
raise NotImplementedError, 'Abstract class %s needs %s.' % (
class_.__name__, ', '.join(class_._abstract_methods))
return super(MustImplement, class_).__call__(*args, **kwargs)
class Abstract(object):
'''A class to stick anywhere in an inheritance chain'''
__metaclass__ = MustImplement
def notimplemented(method):
'''A decorator for those who prefer the parameters declared.'''
return NotImplemented
if __name__ == '__main__':
class SomeBase(dict):
__metaclass__ = MustImplement
needed = also_needed = NotImplemented
class Derived(SomeBase):
def needed(x): return 3
class Further(Derived):
def also_needed(x): return 1
class SomethingMissing(object):
@notimplemented
def function(): return 23
class UsesMissing(SomethingMissing, Abstract): pass
f = Further() # works
try:
d = Derived()
except NotImplementedError, e:
print 'Got expected exception:', e
else:
raise ValueError, 'Did not get expected exception'
s = SomethingMissing() # works
try:
u = UsesMissing()
except NotImplementedError, e:
print 'Got expected exception:', e
else:
raise ValueError, 'Did not get expected exception'
--Scott David Daniels
[EMAIL PROTECTED]
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