__getitem__ and slices

[Daniel Silva]

The Python Reference Manual states:

"The following methods can be defined to implement container objects. ...
The first set of methods is used either to emulate a sequence or to emulate
a mapping; the difference is that for a sequence, the allowable keys should
be the integers k for which 0 <= k < N where N is the length of the
sequence, or slice objects, which define a range of items. ...

__getitem__(self, key)
Called to implement evaluation of self[key]. For sequence types, the
accepted keys should be integers and slice objects."

http://www.python.org/doc/current/ref/sequence-types.html


This code, however, fails:

>>> [1,2,3].__getitem__(slice(1,2))
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in ?
    [1,2,3].__getitem__(slice(1,2))
TypeError: an integer is required

whereas this works:
>>> [1,2,3][1:2]
[2]


It was my understanding that primary[start:stop] would be translated into
primary.__getitem__(slice(start,stop)).  Either the List __getitem__ method
or the documentation for sequences seems incorrect.


- Daniel