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3.3.4 Emulating sequence and mapping types

The following methods can be defined to emulate sequence or mapping objects. 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()`, `copy()`, and `update()` behaving similar to those for Python's standard dictionary objects; mutable sequences should provide methods `append()`, `count()`, `index()`, `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.

`__len__` (self)
  Called to implement the built-in function `len()`. Should return the length of the object, an integer `>=` 0. Also, an object that doesn't define a `__nonzero__()` method and whose `__len__()` method returns zero is considered to be false in a Boolean context.

`__getitem__` (self, key)
  Called to implement evaluation of `self[key]`. For sequence types, the accepted keys should be integers and slice objects. Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the `__getitem__()` method. If key is of an inappropriate type, `TypeError` may be raised; if of a value outside the set of indexes for the sequence (after any special interpretation of negative values), `IndexError` should be raised. Note: `for` loops expect that an `IndexError` will be raised for illegal indexes to allow proper detection of the end of the sequence.

`__setitem__` (self, key, value)
  Called to implement assignment to `self[key]`. Same note as for `__getitem__()`. This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper key values as for the `__getitem__()` method.

`__delitem__` (self, key)
  Called to implement deletion of `self[key]`. Same note as for `__getitem__()`. This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper key values as for the `__getitem__()` method.