[Tutor] abc module question

[Albert-Jan Roskam]

Hi,

I was playing with abc for the first time (never ever had any use for it). It works nicely with methods and properties.
But is there also a way to enforce that class Concrete has an attribute 'foo' of type 'list'? I could of course write a 'foo' property, but I was hoping there was a direct way,

import sys
import abc

class Abstract(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod    
    def __init__(self):
        pass
    @abc.abstractmethod
    def __getitem__(self, key):
        raise NotImplementedError("__getitem__ method required!")
    @abc.abstractproperty
    def encoding(self):
        raise NotImplementedError("encoding property required!")
    @abc.abstractproperty
    def foo(self):
        assert isinstance(self.foo, list)

class Concrete(Abstract):
    def __init__(self):
        self.foo = []  # it could be solved by making foo a property
    def __getitem__(self, key):
        pass
    @property
    def encoding(self):
        return "some encoding"

c = Concrete()

Traceback (most recent call last):
  File "C:\TEMP\abc_foo.py", line 46, in <module>
    c = Concrete()
TypeError: Can't instantiate abstract class Concrete with abstract methods foo

Regards,

Albert-Jan

PS: Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit (Intel)] on win32


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