Detailed explanation of the __getitem__ method in Python and the slicing operation of the slice object

Although the Fib instance can be used in for loops and looks a bit like a list, it is still not possible to use it as a list. For example, take the 5th element:

>>> Fib()[5]
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'Fib' object does not support indexing

To behave like a list and retrieve elements by subscript, you need to implement the __getitem__() method:

class Fib(object):
  def __getitem__(self, n):
    a, b = 1, 1
    for x in range(n):
      a, b = b, a + b
    return a

Now, you can access any item in the sequence by pressing the subscript:

>>> f = Fib()
>>> f[0]
1
>>> f[1]
1
>>> f[2]
2
>>> f[3]
3
>>> f[10]
89
>>> f[100]
573147844013817084101

slice object and __getitem__

To make instances of a class use subscripts like a list, you can set the __getitem__ method. For example:

class _List(object):

  def __getitem__(self, key):
    print key

l = _List()
l[3]  # print 3

But if you want to use slicing operation

l[1:4] # print slice(1, 4, None)

A slice object will be created for slicing. You can check the specific operation through help(slice).

a = slice(1, 4, None)

range(5)[a] # print [1, 2, 3]

More abundant operations

class _List(object):
  
  def __init__(self, _list):
    self._list = _list

  def __getitem__(self, key):
    if isinstance(key, int):
      return self._list[key]
    elif isinstance(key, slice):
      reutrn self.__class__(self._list[key])

if __name__ == '__main__':
  c = _List(range(10))
  b = c[1:5]
  print b[3] # print 4

If the key is an integer, return the list element. If it is a slice object, create an instance and return it.