# generic
class A(object):
    def __iadd__(self, other):
        return NotImplemented

class B(object):
    def __radd__(self, other):
        return 123

a = A()
b = B()

a += b
print a


# lists
class NotList(object):
    def __init__(self, iterable=None):
        self.l = iterable is not None and list(iterable) or list()

    def extend(self, iterable):
        self.l.extend(iterable)

    def __iter__(self):
        return iter(self.l)

    def __radd__(self, other):
        print "__radd__"
        if not isinstance(other, (list, NotList)):
            return NotImplemented
        return NotList(list(other) + self.l)

    def __repr__(self):
        return 'NotList(%r)' % self.l

l_too = l = list('a')
nl = NotList([1,2,3])

l += nl

print 'type(l) is NotList', type(l) is NotList
print 'type(l_too) is list', type(l_too) is list

# $ py23 list_iadd.py
# 123
# __radd__
# type(l) is NotList True
# type(l_too) is list True

# $ py24 list_iadd.py
# 123
# __radd__
# type(l) is NotList True
# type(l_too) is list True

# $ py25 list_iadd.py
# 123
# __radd__
# type(l) is NotList True
# type(l_too) is list True

# $ jython list_iadd.py  # 2.3a0 java1.5.0_09
# NotImplemented
# type(l) is NotList False
# type(l_too) is list True