How can I create customized classes that have similar properties as 'str'?

[Bruno Desthuilliers]

Licheng Fang a écrit :
> I mean, all the class instances that equal to each other should be
> reduced into only one instance, which means for instances of this
> class there's no difference between a is b and a==b.

Here's a Q&D attempt - without any garantee, and to be taylored to your 
needs.

_values = {}    #id(instance) => value mapping
_instances = {} #hash(value) => instance mapping

class Value(object):
   def __new__(cls, value):
     try:
       return _instances[hash(value)]
     except KeyError:
       instance = object.__new__(cls)
       _values[id(instance)] = value
       _instances[hash(value)] = instance
       return instance

   @apply
   def value():
     def fget(self):
       return _values[id(self)]
     def fset(self, ignore):
       raise AttributeError("%s.value is read only" % type(self))
     def fdel(self):
       raise AttributeError("%s.value is read only" % type(self))
     return property(**locals())


HTH