Immutable types allowing subclasses in Python

Question

I want to have immutable types that can, ideally, understand their own hashing and equality, but can be easily subclassed. I started using namedtuple:

class Command(namedtuple('Command', 'cmd_string')):

  def valid_msg(msg):
    return True

  def make_command(msg):
    if self.valid_msg(msg):
      return '%s:%s' % (self.cmd_string, msg)
    else:
      raise ValueError(INVALID_MSG)

... but it does not lend itself to a subclass. Direct subclassification means that the name of the tuple remains unchanged (for printing ... not such a big deal), but more importantly, you cannot add fields:

class LimitedLengthCommand(Command):

  # I want to have self.length! Where does it go?

  def valid_msg(msg):
    return len(msg) <= self.length

Just creating another named tuple (as per the docs) means that I don't inherit any methods!

What is the easiest and easiest way to do something like this? I intend to have several subclasses Command(e.g. hex literals, 1-or-0, etc.), but nothing complicated. Playing a good game with multiple inheritance is not essential.

Answer 1

, , ( ). , , , . , namedtuple . namedtuple , namedtuple . abc.ABCMeta, . :

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__)

:

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:
            # 'new' class
            methods = clsdict
        else:
            # we're 'inheriting' from an existing class
            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

- .

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))

try:
   d =  {b: 1}        # No problems
except TypeError:
    print "Cant put it in a dict"
else:
    print "Can put it in a dict"

, . , , __getattr__, . - baseclass , , .