From 984ce40832609fabb04963518de593b0d17c4944 Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Wed, 15 Sep 2010 00:12:05 +0000 Subject: Clarify where support for negative indices fall in the language hierarchy. --- Doc/reference/expressions.rst | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-) diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst index dff9cc3..c7dd3fc 100644 --- a/Doc/reference/expressions.rst +++ b/Doc/reference/expressions.rst @@ -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. .. index:: single: character -- cgit v0.12