Remove duplicated paragraph.

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Georg Brandl 2008-05-12 16:33:11 +00:00
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@ -7,72 +7,6 @@ Data Structures
This chapter describes some things you've learned about already in more detail,
and adds some new things as well.
.. _tut-tuples:
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-morelists:
More on Lists