class ld( list):
     def __init__(self, x):
          list.__init__(self, x)
          self.__init_check()

     def __init_check(self):
          for record in self:
               if isinstance( record, dict) and "a" in record:
                    pass
               else:
                     raise TypeError("not all entries are dictionaries or have the key 'a'")
          return

tt = ld( [{"a": 1, "b":2}, {"a":4}, {"a":6, "c":67}])
type( tt)

ld( [{"w":1}])
ld( [1,2,3])

type( tt[:2])

class ld( list):
    def __init__(self, x, safe=True):
        list.__init__(self, x)
        self.__init_check( safe)

    def __init_check(self, is_safe):
        if not is_safe:
            return
        for record in self:
            if isinstance( record, dict) and "a" in record:
                pass
            else:
                raise TypeError("not all entries are dictionaries or have the key 'a'")
        return

    def __getslice__(self, i, j):
        return ld( list.__getslice__( self, i, j), safe=False)

def make_verified_list(items):
    """
    :type items: list[object]
    :rtype: list[dict]
    """
    new_list = []
    for item in items:
        if not verify_item(item):
             raise new InvalidItemError(item)
        new_list.append(item)
    return new_list

def verify_item(item):
    """
    :type item: object
    :rtype: bool
    """
    return isinstance(item, dict) and "a" in item