How to define the getitem class to handle both plain indexes and slicing?
Slice objects gets automatically created when you use a colon in the subscript notation - and that is what is passed to __getitem__
. Use isinstance
to check if you have a slice object:
from __future__ import print_function
class Sliceable(object):
def __getitem__(self, subscript):
if isinstance(subscript, slice):
# do your handling for a slice object:
print(subscript.start, subscript.stop, subscript.step)
else:
# Do your handling for a plain index
print(subscript)
Say we were using a range object, but we want slices to return lists instead of new range objects (as it does):
>>> range(1,100, 4)[::-1]
range(97, -3, -4)
We can't subclass range because of internal limitations, but we can delegate to it:
class Range:
"""like builtin range, but when sliced gives a list"""
__slots__ = "_range"
def __init__(self, *args):
self._range = range(*args) # takes no keyword arguments.
def __getattr__(self, name):
return getattr(self._range, name)
def __getitem__(self, subscript):
result = self._range.__getitem__(subscript)
if isinstance(subscript, slice):
return list(result)
else:
return result
r = Range(100)
We don't have a perfectly replaceable Range object, but it's fairly close:
>>> r[1:3]
[1, 2]
>>> r[1]
1
>>> 2 in r
True
>>> r.count(3)
1
To better understand the slice notation, here's example usage of Sliceable:
>>> sliceme = Sliceable()
>>> sliceme[1]
1
>>> sliceme[2]
2
>>> sliceme[:]
None None None
>>> sliceme[1:]
1 None None
>>> sliceme[1:2]
1 2 None
>>> sliceme[1:2:3]
1 2 3
>>> sliceme[:2:3]
None 2 3
>>> sliceme[::3]
None None 3
>>> sliceme[::]
None None None
>>> sliceme[:]
None None None
Python 2, be aware:
In Python 2, there's a deprecated method that you may need to override when subclassing some builtin types.
From the datamodel documentation:
object.__getslice__(self, i, j)
Deprecated since version 2.0: Support slice objects as parameters to the __getitem__()
method. (However, built-in types in CPython currently still implement __getslice__()
. Therefore, you have to override it in derived classes when implementing slicing.)
This is gone in Python 3.