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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 |