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-rw-r--r--Doc/ref/ref3.tex42
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diff --git a/Doc/ref/ref3.tex b/Doc/ref/ref3.tex
index 6648d27..905a001 100644
--- a/Doc/ref/ref3.tex
+++ b/Doc/ref/ref3.tex
@@ -1173,6 +1173,48 @@ involving extended slice notation, or in absence of the slice methods,
\method{__getitem__()}, \method{__setitem__()} or \method{__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
+\method{__getitem__()}, \method{__setitem__()} and \method{__delitem__()}
+support slice objects as arguments):
+
+\begin{verbatim}
+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):]
+ ...
+\end{verbatim}
+
+Note the calls to \function{max()}; these are actually necessary due
+to the handling of negative indices before the
+\method{__*slice__()} methods are called. When negative indexes are
+used, the \method{__*item__()} methods receive them as provided, but
+the \method{__*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 \method{__*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 \method{__*item__()} methods.
+Calling \code{max(0, i)} conveniently returns the proper value.
+
The membership test operators (\keyword{in} and \keyword{not in}) are
normally implemented as iteration loop through the sequence. However,
sequence objects can supply the following special method with a more