readonly class attribute ?
bruno modulix
onurb at xiludom.gro
Tue Mar 15 14:21:19 EST 2005
Hi
How can I make a *class* attribute read-only ?
The answer must be pretty obvious but I just can't find it (it's late
and I've spent all day on metaclasses, descriptors and the like, which,
as fun as it is, may have side-effects on intellectual abilities...)
*The context:*
# library code
class AbstractBaseClass(object):
# snip some stuff here,
# a part of it depending on the derived classes
# defining class attribute class_private_attrib
# client code
class SubClass(AbstractBaseClass):
class_private_attrib = "my private attrib"
# snip
*What I'm looking for: (if possible)*
>>SubClass.class_private_attrib
"my private attrib"
>>SubClass.class_private_attrib = "toto"
AttributeError : SubClass.class_private_attrib is read only
>>s = SubClass()
>>s.class_private_attribute = "toto"
AttributeError : SubClass.class_private_attrib is read only
*What I've tried: (simplified as possible)*
class ReadOnlyDescriptor(object):
def __init__(self, name, initval=None):
self._val = initval
self._name = name
def __get__(self, obj, objtype):
print 'Retrieving', self._name
return self._val
def __set__(self, obj, val):
raise AttributeError, \
"%s.%s is ReadOnly" % (obj.__class.__.__name__, self._name)
class SubClass(object):
class_private_attrib = ReadOnlyDescriptor("class_private_attrib",
"my private attrib")
# snip
*What i get:*
>>SubClass.class_private_attrib
Retrieving class_private_attrib
"my private attrib"
>>SubClass.class_private_attrib = "toto"
>>SubClass.class_private_attrib
"toto"
>>SubClass.__dict__['class_private_attrib']
"toto"
>> s = SubClass()
>> s.class_private_attrib
"toto" # of course :(
*What I understand:*
Ok, I've re-read the manual, noticed that data descriptors __set__()
method was only called when an instance attribute is set (which is
obvious from the prototypes of the methods). My solution is plain wrong
and I should have guess without ever trying. duh :(
Now please have mercy, you Noble Pythoneers : what's the trick to
prevent client code to accidentally mess with the class's dict ? (most
client code - apart from subclass definitions - shouldn't even bother
about the existence of this attribute, it's there for the library
internal usage)
NB : in the real code I'm also messing with the AbstractBaseClass's
meta_class for other stuff (so it's not a problem if the solution
involves metaclasses), but I've tested with the simplified example
above, which exhibits the same problem.
TIA
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb at xiludom.gro'.split('@')])"
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