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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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3.4.6 Additional methods for emulation of sequence types

The following optional methods can be defined to further emulate sequence objects. Immutable sequences should at most only define __getslice__(); mutable sequences might define all three methods.

__getslice__(self, i, j)
This is deprecated in Python 2.0. Support slice objects as parameters to the __getitem__() method. Called to implement evaluation of self[i:j]. The returned object should be of the same type as self. Note that missing i or j in the slice expression are replaced by zero or sys.maxint, respectively. If negative indexes are used in the slice, the length of the sequence is added to that index. If the instance does not implement the __len__() method, an AttributeError is raised. No guarantee is made that indexes adjusted this way are not still negative. Indexes which are greater than the length of the sequence are not modified. If no __getslice__() is found, a slice object is created instead, and passed to __getitem__() instead.
__setslice__(self, i, j, sequence)
Called to implement assignment to self[i:j]. Same notes for i and j as for __getslice__(). This method is deprecated. If no __setslice__() is found, or for extended slicing of the form self[i:j:k], a slice object is created, and passed to __setitem__(), instead of __setslice__() being called.
__delslice__(self, i, j)
Called to implement deletion of self[i:j]. Same notes for i and j as for __getslice__(). This method is deprecated. If no __delslice__() is found, or for extended slicing of the form self[i:j:k], a slice object is created, and passed to __delitem__(), instead of __delslice__() being called.

Notice that these methods are only invoked when a single slice with a single colon is used, and the slice method is available. For slice operations involving extended slice notation, or in the absence of slice methods, __getitem__(), __setitem__() or __delitem__() is called with a slice object as argument.

The following example demonstrate how to make your program or module compatible with earlier versions of Python (assuming that methods __getitem__(), __setitem__() and __delitem__() support slice objects as arguments):

    class MyClass:
        ...
        def __getitem__(self, index):
            ...
        def __setitem__(self, index, value):
            ...
        def __delitem__(self, index):
            ...
    
        if sys.version_info < (2, 0):
            # They won't be defined if version
            # is at least 2.0 final
    
            def __getslice__(self, i, j):
                return self[max(0, i):max(0, j):]
            def __setslice__(self, i, j, seq):
                self[max(0, i):max(0, j):] = seq
            def __delslice__(self, i, j):
                del self[max(0, i):max(0, j):]
        ...

Note the calls to max(); these are necessary because of the handling of negative indices before the __...slice__() methods are called. When negative indexes are used, the __...item__() methods receive them as provided, but the __...slice__() methods get a "cooked" form of the index values. For each negative index value, the length of the sequence is added to the index before calling the method (which may still result in a negative index); this is the customary handling of negative indexes by the built-in sequence types, and the __...item__() methods are expected to do this as well. However, since they should already be doing that, negative indexes cannot be passed in; they must be constrained to the bounds of the sequence before being passed to the __...item__() methods. Calling max(0, i) conveniently returns the proper value.

ISBN 0954161785Python Language Reference ManualSee the print edition