class WithProtection(object):
'''
WithProtection can be used as a base class for all classes that
want true protection of user defined attributes mangled with a
single underscore "_".
'''
def __new__(cls, *args, **kwd):
obj = object.__new__(cls)
cls.__init__(obj, *args, **kwd)
def __getattr__(self, name):
attr = getattr(obj, name)
if name.startswith("_"):
if name.startswith("__") and name.endswith("__"):
return attr
else:
return attr
raise AttributeError, "Attribute %s is not public."%name
def __setattr__(self, name, value):
if name.startswith("__") and name.endswith("__"):
raise AttributeError,"magic attributes are write protected."
attr = getattr(obj, name)
cls.__setattr__(self, name, value)
# Magic methods defined by cls must be copied to Proxy.
# Delegation using __getattr__ is not possible.
def is_own_magic(cls, name, without=[]):
return name not in without and\
name.startswith("__") and name.endswith("__")
Proxy = type("WithProtection(%s)"%cls.__name__,(),{})
for name in dir(cls):
if is_own_magic(cls,name, without=dir(Proxy)):
try:
setattr(Proxy, name, getattr(obj,name))
except TypeError:
pass
Proxy.__getattr__ = __getattr__
Proxy.__setattr__ = __setattr__
proxy = Proxy()
return proxy
#
# Example
#
class Counter(WithProtection):
def __init__(self, start=0):
self._n = start
def inc(self):
self._n+=1
return self._n
def dec(self):
self._n-=1
return self._n
def value(self):
return self._n
#
# Enhanced example using inheritance
#
class SteppedCounter(Counter):
def __init__(self, start=0, step=1):
super(SteppedCounter,self).__init__(start=start)
self.step = step
def inc(self):
self._n+=self.step
return self._n
def __add__(self, k):
return self._n+k
def dec(self):
self._n-=self.step
return self._n
def steps(self):
return self._n/self.step
################################################################
# Python Session
################################################################
>>> sc = SteppedCounter()
>>> sc.step
1
>>> sc._n
Traceback (most recent call last):
....
AttributeError: Attribute _n is not public.
>>> sc.inc()
1
>>> sc+8
9
