Python: Implementing slicing in __getitem__

Question

I am trying to implement slice functionality for a class I am making that creates a vector representation.

I have this code so far, which I believe will properly implement the slice but whenever I do a call like v[4] where v is a vector python returns an error about not having enough parameters. So I am trying to figure out how to define the getitem class to handle both plain indexes and slicing.

def __getitem__(self, start, stop, step):
    indx = start
    if stop == None:
        end = start + 1
    else:
        end = stop
    if step == None:
        stride = 1
    else:
        stride = step
    return self.__data[indx:end:stride]

Answer 1

The __getitem__() method will receive a slice object when the object is sliced. Simply look at the start, stop, and step members of the slice object in order to get the components for the slice.

>>> class C(object):
...   def __getitem__(self, val):
...     print val
... 
>>> c = C()
>>> c[3]
3
>>> c[3:4]
slice(3, 4, None)
>>> c[3:4:-2]
slice(3, 4, -2)
>>> c[():1j:'a']
slice((), 1j, 'a')

Answer 2

The correct way to do this is to have __getitem__ take one parameter, which can either be a number, or a slice object.

See:

http://docs.python.org/library/functions.html#slice http://docs.python.org/reference/datamodel.html#object.__getitem__