import collections as co
import abc
class ImmutableMeta(abc.ABCMeta):
_classes = {}
def __new__(meta, clsname, bases, clsdict):
attributes = clsdict.pop('_attributes_')
if bases[0] is object:
# We're creating a class 'from scratch'
methods = clsdict
else:
# we're 'inheriting' from an existing class.
# Pull the base class's attributes and methods from the registry and
# combine them with the new class's attributes and methods.
base = bases[0]
attributes = meta._classes[base]['attributes'] + ' ' + attributes
base_methods = meta._classes[base]['methods'].copy()
base_methods.update(clsdict)
methods = base_methods
# construct the actual base class and create the return class
new_base = co.namedtuple(clsname + 'Base', attributes)
cls = super(ImmutableMeta, meta).__new__(meta, clsname, (new_base,),
methods)
# register the data necessary to 'inherit' from the class
# and make sure that it passes typechecking
meta._classes[cls] = {'attributes': attributes,
'methods': methods}
if bases[0] is not object:
base.register(cls)
return cls
# Demo
class Foo(object):
__metaclass__ = ImmutableMeta
_attributes_ = 'a b'
def sayhi(self):
print "Hello from {0}".format(type(self).__name__)
class Bar(Foo):
_attributes_ = 'c'
def saybye(self):
print "Goodbye from {0}".format(type(self).__name__)
a = Foo(1, 2)
a.sayhi()
b = Bar(1, 2, 3)
b.sayhi() # 'inherited' from class Foo
b.saybye()
try:
b.c = 1 # will raise an AttributeError
except AttributeError:
print "Immutable"
print "issubclass(Bar, Foo): {0}".format(issubclass(Bar, Foo))
# We get this for free with abc
try:
d = {b: 1} # No problems
except TypeError:
print "Cant put it in a dict"
else:
print "Can put it in a dict"
Diff to Previous Revision
--- revision 3 2010-10-11 23:28:30
+++ revision 4 2010-10-11 23:32:31
@@ -13,8 +13,8 @@
methods = clsdict
else:
# we're 'inheriting' from an existing class.
- # Pull the base class attributes and methods from the registry and
- # combine them with the new classes attributes and methods.
+ # Pull the base class's attributes and methods from the registry and
+ # combine them with the new class's attributes and methods.
base = bases[0]
attributes = meta._classes[base]['attributes'] + ' ' + attributes
base_methods = meta._classes[base]['methods'].copy()
