How to implement a minimal class that behaves like a sequence in Python?

I am looking for a sample minimal example of a class that mimics an immutable sequence in Python.

class MySequence()
    ...


a = MySequence()

len(a)

for i in a:
    pass

a[0]

What are the methods that must be implemented?

Answer

If you just want to be able to iterate over your sequence, you just need to implement the __iter__ method returning an iterable. The easiest way to do this is to create a generator using the yield statement.

class MySequence(object):
    def __iter__(self):
        yield 1
        yield 2
        yield 3


for x in MySequence():
    print x # prints 1, then 2, then 3

However, this will not enable things like MySequence()[1]. For that you need to implement the __getitem__ method, and should probably implement __len__ as well.

class MySequence(object):
    def __len__(self):
        return 3


    def __getitem__(self, key):
        if key == 0:
            return 1
        elif key == 1:
            return 2
        elif key == 2:
            return 3
        else:
            raise IndexError()


s = new MySequence()

for i in range(len(s)):
    print s[i] # prints 1, then 2, then 3

for x in s:
    print x # prints 1, then 2, then 3

Notice that I omitted __iter__. As long as __getitem__ raises an IndexError when you try to get a value that's out-of-bounds, Python can use it for iteration. (I could still include __iter__ if I wanted to be clearer, or wanted non-standard iteration behaviour.)