3.3.5 Additional methods for emulation of sequence types

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

__getslice__ (self, i, j): Deprecated since release 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, a slice object is created instead, and passed to __setitem__() instead.

__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, a slice object is created instead, and passed to __delitem__() instead.

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 absence of the 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 actually necessary due to 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 be constrained to the bounds of the sequence before being passed to the __*item__() methods. Calling max(0, i) conveniently returns the proper value.

The membership test operators (in and not in) are normally implemented as iteration loop through the sequence. However, sequence objects can supply the following special method with a more efficient implementation:

__contains__ (self, item): Called to implement membership test operators. Should return true if item is in self, false otherwise.

Ver Sobre este documento... para obtener información sobre sugerencias.