Written by Gaoping Huang
on 
Tags: Python

Implementing slicing in __getitem__

Sometimes, we need to implement slicing functionality in our own class. Below is an example using slicing to get the installed Python version (copied from six.py, a Python2 and 3 compatibility library):

import sys
PY34 = sys.version_info[0:2] >= (3,4)

Here, sys.version_info contains (major, minor, micro, …). By using [0:2], we get a tuple (major, minor), such as (2, 7) for Python2.7.3.

To support slicing, we need to use object.__getitem__(self, key) method. The key could be an integer or slice objects. For example,

>>> class A(object):
...   def __getitem__(self, key):
...     print key
... 
>>> a = A()
>>> a[0]
0  # an integer
>>> a[0:2]
slice(0, 2, None)  # a slice object
>>> a[0:4:2]
slice(0, 4, 2)  # a slice object of (start, stop, and step)
>>> a[0,4,2]
(0, 4, 2)  # a tuple as index, which is out of our scope today

That is to say, a call a[0:2] is equivalent to a[slice(0,2,None)].

With this in mind, we can customize our slicing as below:

class Seq(object):
  def __init__(self, seq):
      self._seq = seq

  def get_value(self, i):
      pass

  def __getitem__(self, key):
      if isinstance(key, slice):
          start, stop, step = key.indices(len(self))
          return Seq([self[i] for i in range(start, stop, step)])
      elif isinstance(key, int):
          return self.get_value(key)
      elif isinstance(key, tuple):
          raise NotImplementedError, 'Tuple as index'
      else:
          raise TypeError, 'Invalid argument type: {}'.format(type(key))

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