class Sheep（object）：
  def get_size(self):
    raise NotImplementedError

import abc
class Sheep(object):
  __metaclass__ = abc.ABCMeta
  
  @abc.absractmethod
  def get_size(self):
    return

import abc 

class A(object):
  __metaclass__ = abc.ABCMeta

  @abc.abstractmethod
  def load(self, input):
    return 

  @abc.abstractmethod
  def save(self, output, data):
    return

class B(object):
  
  def load(self, input):
    return input.read()

  def save(self, output, data):
    return output.write(data)

A.register(B)

if __name__ == '__main__':
  print issubclass(B, A)   # print True
  print isinstance(B(), A)  # print True

class C(A):

  def load(self, input):
    return input.read()

  def save(self, output, data):
    return output.write(data)
    
if __name__ == '__main__':
  print issubclass(C, A)   # print True
  print isinstance(C(), A)  # print True

for sc in A.__subclasses__():
  print sc.__name__

# print C

import abc

class A(object):
  __metaclass__ = abc.ABCMeta

  @abc.abstractmethod
  def say(self):
    return 'say yeah'

  @classmethod
  def __subclasshook__(cls, C):
    if cls is A:
      if any("say" in B.__dict__ for B in C.__mro__):
        return True
    return NotTmplementd

class B(object):
  def say(self):
    return 'hello'

print issubclass(B, A)   # True
print isinstance(B(), A)  # True
print B.__dict__      # {'say': <function say at 0x7f...>, ...}
print A.__subclasshook__(B) # True

class D(A):
  def save(self, output, data):
    return output.write(data)

if __name__ == '__main__':
  print issubclass(D, A)   # print True
  print isinstance(D(), A)  # raise TypeError

import abc 
from cStringIO import StringIO

class A(object):
  __metaclass__ = abc.ABCMeta

  @abc.abstractmethod
  def retrieve_values(self, input):
    pirnt 'base class reading data'
    return input.read()


class B(A):

  def retrieve_values(self, input):
    base_data = super(B, self).retrieve_values(input)
    print 'subclass sorting data'
    response = sorted(base_data.splitlines())
    return response

input = StringIO("""line one
line two
line three
""")

reader = B()
print reader.retrieve_values(input)

base class reading data
subclass sorting data
['line one', 'line two', 'line three']

import abc

class A(object):
  __metaclass__ = abc.ABCMeta

  @abc.abstractproperty
  def value(self):
    return 'should never get here.'

class B(A):
  
  @property
  def value(self):
    return 'concrete property.'

try:
  a = A()
  print 'A.value', a.value
except Exception, err:
  print 'Error: ', str(err)

b = B()
print 'B.value', b.value

Error: ...
print concrete property

import abc

class A(object):
  __metaclass__ = abc.ABCMeta

  def value_getter(self):
    return 'Should never see this.'

  def value_setter(self, value):
    return 

  value = abc.abstractproperty(value_getter, value_setter)

class B(A):
  
  @abc.abstractproperty
  def value(self):
    return 'read-only'

class C(A):
  _value = 'default value'

  def value_getter(self):
    return self._value

  def value_setter(self, value):
    self._value = value

  value = property(value_getter, value_setter)

try:
  a = A()
  print a.value
except Exception, err:
  print str(err)

try:
  b = B()
  print b.value
except Exception, err:
  print str(err)

c = C()
print c.value

c.value = 'hello'
print c.value

error: ...
error: ...
print 'default value'
print 'hello'

import abc

class A(object):
  __metaclass__ = abc.ABCMeta

  @abc.abstractproperty
  def value(self):
    return 'should never see this.'

  @value.setter
  def value(self, _value):
    return 

class B(A):
  _value = 'default'

  @property
  def value(self):
    return self._value

  @value.setter
  def value(self, _value):
    self._value = _value

b = B()
print b.value    # print 'default'

b.value = 'hello'
print b.value    # print 'hello'