mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-11-04 03:44:55 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 9

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

Browse source
This commit is contained in:
Raymond Hettinger 2010-09-15 00:09:26 +00:00
parent 476a31ef1e
commit f77c1d6adb
1 changed files with 12 additions and 5 deletions
Download patch file Download diff file Expand all files Collapse all files

17
Doc/reference/expressions.rst
View file

@ -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 value in the mapping that corresponds to that key. (The expression list is a
tuple except if it has exactly one item.) tuple except if it has exactly one item.)
If the primary is a sequence, the expression (list) must evaluate to an integer. 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, or a slice (as discussed in the following section).
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 The formal syntax makes no special provision for negative indices in
subscription selects the item whose index is that value (counting from zero). 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:: .. index::
single: character single: character