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Clarify where support for negative indices fall in the language hierarchy.

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Raymond Hettinger 2010-09-15 00:09:26 +00:00
parent 476a31ef1e
commit f77c1d6adb
1 changed files with 12 additions and 5 deletions
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17
Doc/reference/expressions.rst
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@ -518,11 +518,18 @@ whose value is one of the keys of the mapping, and the subscription selects the
value in the mapping that corresponds to that key. (The expression list is a
tuple except if it has exactly one item.)
If the primary is a sequence, the expression (list) must evaluate to an integer.
If this value is negative, the length of the sequence is added to it (so that,
e.g., ``x[-1]`` selects the last item of ``x``.) The resulting value must be a
nonnegative integer less than the number of items in the sequence, and the
subscription selects the item whose index is that value (counting from zero).
If the primary is a sequence, the expression (list) must evaluate to an integer
or a slice (as discussed in the following section).
The formal syntax makes no special provision for negative indices in
sequences; however, built-in sequences all provide a :meth:`__getitem__`
method that interprets negative indices by adding the length of the sequence
to the index (so that ``x[-1]`` selects the last item of ``x``). The
resulting value must be a nonnegative integer less than the number of items in
the sequence, and the subscription selects the item whose index is that value
(counting from zero). Since the support for negative indices and slicing
occurs in the object's :meth:`__getitem__` method, subclasses overriding
this method will need to explicitly add that support.
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