len and getitem

Learn sequence behavior with __len__ and __getitem__.

len and getitem

__len__(self) gives the result of len(obj). __getitem__(self, index) defines obj[index] access. Together they make a class behave like a sequence.

Example: class Stack:\n def __len__(self):\n return len(self.items)\n def __getitem__(self, index):\n return self.items[index]. len(s) gives the count; s[0] gives the first item. This makes the class behave like built-in sequence types.

Sequence-like usage

Classes with __len__ and __getitem__ can be used like a sequence. len(s), s[0], s[-1] and slicing work. This gives a natural interface for collection classes.

Example: in the Stack class, len(s) gives the item count; s[0] returns the bottom item. A student may think it is a list; but really __getitem__ is called. This way the class follows Python idioms.

Exercise Add sequence methods to Stack

Define the Stack class. Add def __init__(self): with self.items = [] in the body. Add def push(self, value): with self.items.append(value). Add def __len__(self): with return len(self.items). Add def __getitem__(self, index): with return self.items[index]. Then call s = Stack(), s.push(1), s.push(2), print(len(s)) and print(s[0]).

Show example solution

# Define the Stack class
class Stack:
    def __init__(self):
        self.items = []
    def push(self, value):
        self.items.append(value)
    def __len__(self):
        return len(self.items)
    def __getitem__(self, index):
        return self.items[index]

s = Stack()
s.push(1)
s.push(2)
print(len(s))
print(s[0])

Test yourself

Match the sequence methods

__len__

len()
__getitem__

obj[index]
len(self.items)

Inner list size
Which method does len(obj) call?
- __size__
- __length__
- __len__
- __count__
Show answer

__len__ — The len() function calls the __len__ method.

Complete the __getitem__ signature

class Stack:
    def __getitem__(self, ____):
        return self.items[index]

Show answer

class Stack:
    def __getitem__(self, index):
        return self.items[index]

Next lesson: __iter__ and __next__

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