Python Cookbook

Overriding a Built-In Method

Credit: Dave Haynes

Problem

You need to wrap (or, in Python 2.2, inherit from) a list or tuple, delegating several operations to it, and want to provide proper slicing (i.e., through the special method _ _getitem_ _).

Solution

In most cases, overriding special methods of built-in objects when you inherit from those objects (or wrap them with automatic delegation, which is not technically an override) poses no special challenge. When inheriting in Python 2.2, you can call the special method of the superclass with the usual unbound-method syntax. When wrapping, use the syntax that is specific to the operation, such as self.data[someindex] for indexing.

Slicing is harder, because while slicing should go through the same special method _ _getitem_ _ as indexing (since Python 2.0), lists and tuples still implement an older approach: the more limited special method _ _getslice_ _ (and similarly for _ _setitem_ _ versus _ _setslice_ _ and _ _delitem_ _ versus _ _delslice_ _). So, you must provide a remedy, normally with a try/except:

class SliceTester:
    def _ _init_ _(self):
        self.data = ['zero', 'one', 'two', 'three', 'four']

    def  _ _getitem_ _(self, indexOrSlice):
        try:
            return self.data[indexOrSlice]
        except TypeError:
            return self.data[indexOrSlice.start:indexOrSlice.stop]

Discussion

When a user-defined class wraps (or, in Python 2.2, inherits from) a list or tuple, it often needs to define the _ _set*_ _ and _ _get*_ _ special methods and delegate part or all of their ...