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| -rw-r--r-- | Doc/reference/datamodel.rst | 9 |
1 files changed, 6 insertions, 3 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index e41697d..afe1381 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -299,14 +299,17 @@ Sequences These represent finite ordered sets indexed by non-negative numbers. The built-in function :func:`len` returns the number of items of a sequence. When the length of a sequence is *n*, the index set contains the numbers 0, 1, -..., *n*-1. Item *i* of sequence *a* is selected by ``a[i]``. +..., *n*-1. Item *i* of sequence *a* is selected by ``a[i]``. Some sequences, +including built-in sequences, interpret negative subscripts by adding the +sequence length. For example, ``a[-2]`` equals ``a[n-2]``, the second to last +item of sequence a with length ``n``. .. index:: single: slicing Sequences also support slicing: ``a[i:j]`` selects all items with index *k* such that *i* ``<=`` *k* ``<`` *j*. When used as an expression, a slice is a -sequence of the same type. This implies that the index set is renumbered so -that it starts at 0. +sequence of the same type. The comment above about negative indexes also applies +to negative slice positions. Some sequences also support "extended slicing" with a third "step" parameter: ``a[i:j:k]`` selects all items of *a* with index *x* where ``x = i + n*k``, *n* |