summaryrefslogtreecommitdiffstats
path: root/Doc/tutorial/datastructures.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Doc/tutorial/datastructures.rst')
-rw-r--r--Doc/tutorial/datastructures.rst64
1 files changed, 64 insertions, 0 deletions
diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst
index cf9fea3..206f056 100644
--- a/Doc/tutorial/datastructures.rst
+++ b/Doc/tutorial/datastructures.rst
@@ -350,6 +350,70 @@ is assigned to it). We'll find other uses for :keyword:`del` later.
+Tuples and Sequences
+====================
+
+We saw that lists and strings have many common properties, such as indexing and
+slicing operations. They are two examples of *sequence* data types (see
+:ref:`typesseq`). Since Python is an evolving language, other sequence data
+types may be added. There is also another standard sequence data type: the
+*tuple*.
+
+A tuple consists of a number of values separated by commas, for instance::
+
+ >>> t = 12345, 54321, 'hello!'
+ >>> t[0]
+ 12345
+ >>> t
+ (12345, 54321, 'hello!')
+ >>> # Tuples may be nested:
+ ... u = t, (1, 2, 3, 4, 5)
+ >>> u
+ ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
+
+As you see, on output tuples are always enclosed in parentheses, so that nested
+tuples are interpreted correctly; they may be input with or without surrounding
+parentheses, although often parentheses are necessary anyway (if the tuple is
+part of a larger expression).
+
+Tuples have many uses. For example: (x, y) coordinate pairs, employee records
+from a database, etc. Tuples, like strings, are immutable: it is not possible
+to assign to the individual items of a tuple (you can simulate much of the same
+effect with slicing and concatenation, though). It is also possible to create
+tuples which contain mutable objects, such as lists.
+
+A special problem is the construction of tuples containing 0 or 1 items: the
+syntax has some extra quirks to accommodate these. Empty tuples are constructed
+by an empty pair of parentheses; a tuple with one item is constructed by
+following a value with a comma (it is not sufficient to enclose a single value
+in parentheses). Ugly, but effective. For example::
+
+ >>> empty = ()
+ >>> singleton = 'hello', # <-- note trailing comma
+ >>> len(empty)
+ 0
+ >>> len(singleton)
+ 1
+ >>> singleton
+ ('hello',)
+
+The statement ``t = 12345, 54321, 'hello!'`` is an example of *tuple packing*:
+the values ``12345``, ``54321`` and ``'hello!'`` are packed together in a tuple.
+The reverse operation is also possible::
+
+ >>> x, y, z = t
+
+This is called, appropriately enough, *sequence unpacking*. Sequence unpacking
+requires the list of variables on the left to have the same number of elements
+as the length of the sequence. Note that multiple assignment is really just a
+combination of tuple packing and sequence unpacking!
+
+There is a small bit of asymmetry here: packing multiple values always creates
+a tuple, and unpacking works for any sequence.
+
+.. XXX Add a bit on the difference between tuples and lists.
+
+
.. _tut-sets:
Sets