old_contents: # -*- coding: utf-8 -*-
  '''Abstract base classes.
  
  These are necessary to avoid circular imports between core.py and fields.py.
  '''
  
  
  class FieldABC(object):
      '''Abstract base class from which all Field classes inherit.
      '''
      parent = None
      name = None
  
      def format(self, value):
          raise NotImplementedError
  
      def output(self, key, obj):
          raise NotImplementedError
  
      def __repr__(self):
          return "<{0} Field>".format(self.__class__.__name__)
  
      __str__ = __repr__
  
  
  class SerializerABC(object):
      '''Abstract base class from which all Serializers inherit.'''
  
      @property
      def errors(self):
          raise NotImplementedError
  
      def is_valid(self, fields=None):
          raise NotImplementedError
  
new_contents: # -*- coding: utf-8 -*-
  '''Abstract base classes.
  
  These are necessary to avoid circular imports between core.py and fields.py.
  '''
  import copy
  
  
  class FieldABC(object):
      '''Abstract base class from which all Field classes inherit.
      '''
      parent = None
      name = None
  
      def format(self, value):
          raise NotImplementedError
  
      def output(self, key, obj):
          raise NotImplementedError
  
      def __deepcopy__(self, memo):
          ret = copy.copy(self)
          return ret
  
      def __repr__(self):
          return "<{0} Field>".format(self.__class__.__name__)
  
      __str__ = __repr__
  
  
  class SerializerABC(object):
      '''Abstract base class from which all Serializers inherit.'''
  
      @property
      def errors(self):
          raise NotImplementedError
  
      def is_valid(self, fields=None):
          raise NotImplementedError
  
subject: Speed up deep copy of fields