Entry

Typed list

Jul 5th, 2000 10:03
Nathan Wallace, unknown unknown, Hans Nowak, Snippet 303, Python Snippet Support Team

"""
Packages: basic_datatypes.lists
"""
# typedlist.py
"""Provides a class for lists of a certain type, like parametrized classes
in Eiffel, or template classes in C. While this concept is an extension of
those languages mentioned, it rather seems to be a *restriction* of Python's
capabilities."""
from UserList import UserList
# Based on UserList, so I can reuse most of that class; its subclass must
# only make sure that added values are of the correct type.
class TypedList(UserList):
    def __init__(self, aType):
        UserList.__init__(self)
        self.__type = aType
    def append(self, x):
        if type(x) != self.__type:
            raise TypeError, 'TypedList: can only add objects of %s' % self.__type
        self.data.append(x)
if __name__ == "__main__":
    tl = TypedList(type(3)) # a list of integers, obviously
    # trying to append something else
    try:
        tl.append('Hoi')
    except TypeError, msg:
        print msg
    tl.append(35)   # OK