The following methods can be defined to implement container objects. Containers usually are sequences (such as lists or tuples) or mappings (like dictionaries), but can represent other containers as well. 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. (For backwards compatibility, the method __getslice__ (see below) can also be defined to handle simple, but not extended slices.) It is also recommended that mappings provide the methods keys(), values(), items(), has_key(), get(), clear(), setdefault(), iterkeys(), itervalues(), iteritems(), pop(), popitem(), copy(), and update() behaving similar to those for Python's standard dictionary objects. The UserDict module provides a DictMixin class to help create those methods from a base set of __getitem__, __setitem__, __delitem__, and keys(). Mutable sequences should provide methods append(), count(), index(), extend(), insert(), pop(), remove(), reverse() and sort(), like Python standard list objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods __add__, __radd__, __iadd__, __mul__, __rmul__ and __imul__ described below; they should not define __coerce__ or other numerical operators. It is recommended that both mappings and sequences implement the __contains__ method to allow efficient use of the in operator; for mappings, in should be equivalent of has_key(); for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the __iter__ method to allow efficient iteration through the container; for mappings, __iter__ should be the same as iterkeys(); for sequences, it should iterate through the values.