[Tutor] New Style Classes, __getitem__ and iteration

[Marilyn Davis]

Hi Tutors and Tutees,

I've been teaching Python quite a while and a brilliant student asked a
question that made me realize a big hole in my understanding.

I think it is really magical that, when I define __getitem__ on classic
classes, the built-in iterator uses it.

But, when I make the same class as a new style class, I lose this behavior.

I didn't realize that something was lost in defining a new style class. 
Maybe it's something else that I'm missing.

Thank you for your insights.

Marilyn

p.s.

Here's some code that demonstrates my confusion:

#!/usr/bin/env python

class Circle:

    def __init__(self, data, times):
        """Put the 'data' in a circle that goes around 'times' times."""
        self.data = data
        self.times = times

    def __getitem__(self, i):
        """circle[i] --> Circle.__getitem__(circle, i)."""
        l_self = len(self)
        if i >= self.times * l_self:
            raise IndexError, \
                  "Error raised: Circle object only goes around %d times"\
                  % self.times
        return self.data[i % l_self]

    def __len__(self):
        return len(self.data)

class NewCircle(list):

    def __init__(self, data, times):
        list.__init__(self, data)
        self.times = times

    def __getitem__(self, i):
        l_self = len(self)
        if i >= self.times * l_self:
            raise IndexError, \
                  "Error raised: NewCircle object only goes around %d times"\
                  % self.times
        return list.__getitem__(self, i % l_self)

def main():
    circle = Circle("around", 3)
    print sorted(circle)
    new_circle = NewCircle("around", 3)
    print sorted(new_circle)

main()
"""
OUTPUT:

$ ./circle_question.py
['a', 'a', 'a', 'd', 'd', 'd', 'n', 'n', 'n', 'o', 'o', 'o', 'r', 'r',
'r', 'u', 'u', 'u']
['a', 'd', 'n', 'o', 'r', 'u']
$

"""