Subclass Python list to Validate New Items

Question

I want a python list which represents itself externally as an average of its internal list items, but otherwise behaves as a list. It should raise a TypeError i

Answer 1

There are 7 methods of the list class that add elements to the list and would have to be checked. Here's one compact implementation:

def check_float(x):
    try:
        f = float(x)
    except:
        raise TypeError("Cannot add %s to AverageList" % str(x))

def modify_method(f, which_arg=0, takes_list=False):
    def new_f(*args):
        if takes_list:
            map(check_float, args[which_arg + 1])
        else:
            check_float(args[which_arg + 1])
        return f(*args)
    return new_f

class AverageList(list):
    def __check_float(self, x):
        try:
            f = float(x)
        except:
            raise TypeError("Cannot add %s to AverageList" % str(x))

    append = modify_method(list.append)
    extend = modify_method(list.extend, takes_list=True)
    insert = modify_method(list.insert, 1)
    __add__ = modify_method(list.__add__, takes_list=True)
    __iadd__ = modify_method(list.__iadd__, takes_list=True)
    __setitem__ = modify_method(list.__setitem__, 1)
    __setslice__ = modify_method(list.__setslice__, 2, takes_list=True)