What's the difference between __iter__ and __getitem__?

Question

This happens in Python 2.7.6 and 3.3.3 for me. When I define a class like this

class foo:
    def __getitem__(self, *args):
        print(*args)

Answer 1

To get the result you are expecting, you need to have a data element with limited len and return each in sequence:

class foo:
    def __init__(self):
        self.data=[10,11,12]

    def __getitem__(self, arg):
        print('__getitem__ called with arg {}'.format(arg))
        return self.data[arg]

bar = foo()
for i in bar:
    print('__getitem__ returned {}'.format(i)) 

Prints:

__getitem__ called with arg 0
__getitem__ returned 10
__getitem__ called with arg 1
__getitem__ returned 11
__getitem__ called with arg 2
__getitem__ returned 12
__getitem__ called with arg 3

Or you can signal the end of the 'sequence' by raising IndexError (although StopIteration works as well...):

class foo:
    def __getitem__(self, arg):
        print('__getitem__ called with arg {}'.format(arg))
        if arg>3:
            raise IndexError
        else:    
            return arg

bar = foo()
for i in bar:
    print('__getitem__ returned {}'.format(i))   

Prints:

__getitem__ called with arg 0
__getitem__ returned 0
__getitem__ called with arg 1
__getitem__ returned 1
__getitem__ called with arg 2
__getitem__ returned 2
__getitem__ called with arg 3
__getitem__ returned 3
__getitem__ called with arg 4

The for loop is expecting either IndexError or StopIteration to signal the end of the sequence.