Hi all,
I was wondering if someone can help me understand why __getslice__ has been
deprecated, yet it remains necessary to implement it for simple slices (i:j),
while __getitem__ gets called for extended slices (i:j:k).
The problem with this approach, besides a bit of code duplication, is that
classes which implement slicing must now do runtime type-checking inside
__getitem__. Here's a trivial example:
################################################################
import types
class Vector(list):
def __init__(self,arg,wrap=0):
'''
If arg is an integer, make a new zero vector of length n.
Otherwise, arg must be a python sequence or another vector,
and a new (deep) copy is generated.
'''
if isinstance(arg,int):
list.__init__(self,[0.0]*arg)
else:
list.__init__(self,arg)
def __getitem__(self,key):
"""called for single-element OR slice access"""
if type(key) is types.SliceType:
return Vector(list.__getitem__(self,key))
else:
return list.__getitem__(self,key)
def __getslice__(self,i,j):
"""Deprecated since 2.0, but still called for non-extended slices.
Since extended slices are handled by __getitem__, I'm just deferring
to that one so all actual implementation is done there. Why this is
not the default (since they deprecated __getslice__) is beyond me."""
return self.__getitem__(slice(i,j))
print 'a is a vector'
a = Vector(5)
print a
print type(a)
print
print 'b is a slice of a'
b = a[1:3]
print b
print type(b)
print
print 'c is an element of a'
c = a[1]
print c
print type(c)
################################################################
What bugs me is that typecheck for slicing which seems to be necessary inside
of the __getitem__ method. I have the feeling that the code would be much
cleaner if I could simply use __getslice__ for slices and __getitem__ for
items, and that bundling the two in this hybrid mode is reall ugly and
unpythonic. Am I just missing something?
Thanks for any help,
f
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