Python Programming/Tuples

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[edit] About tuples in Python

A tuple in Python is much like a list except that it is immutable (unchangeable) once created. They are generally used for data which should not be edited.

[edit] Tuple notation

Tuples may be created directly or converted from lists. Generally, tuples are enclosed in parenthesis.

>>> l = [1, 'a', [6, 3.14]]
>>> t = (1, 'a', [6, 3.14])
>>> t
(1, 'a', [6, 3.1400000000000001])
>>> tuple(l)
(1, 'a', [6, 3.1400000000000001])
>>> t == tuple(l)
True
>>> t == l
False

A one item tuple is created by a item in parens followed by a comma:

>>> t = ('A single item tuple',)
>>> t
('A single item tuple',)

Also, tuples will be created from items separated by commas.

>>> t = 'A', 'tuple', 'needs', 'no', 'parens'
>>> t
('A', 'tuple', 'needs', 'no', 'parens')

[edit] Packing and Unpacking

You can also perform multiple assignment using tuples.

>>> article, noun, verb, adjective, direct_object = t
>>> noun
'tuple'

Note that either, or both sides of an assignment operator can consist of tuples.

>>> a, b = 1, 2
>>> b
2

Assigning a tuple to a several different variables is called "tuple unpacking," while assigning multiple values to a tuple in one variable is called "tuple packing." When unpacking a tuple, or performing multiple assignment, you must have the same number of variables being assigned to as values being assigned.

[edit] Operations on tuples

These are the same as for lists except that we may not assign to indices or slices, and there is no "append" operator.

>>> a = (1, 2)
>>> b = (3, 4)
>>> a + b
(1, 2, 3, 4)
>>> a
(1, 2)
>>> b
(3, 4)
>>> print a.append(3)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'append'
>>> a
(1, 2)
>>> a[0] = 0
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: object does not support item assignment
>>> a
(1, 2)

For lists we would have had:

>>> a = [1, 2]
>>> b = [3, 4]
>>> a + b
[1, 2, 3, 4]
>>> a
[1, 2]
>>> b
[3, 4]
>>> a.append(3)
>>> a
[1, 2, 3]
>>> a[0] = 0
>>> a
[0, 2, 3]

[edit] Tuple Attributes

Length: Finding the length of a tuple is the same as with lists; use the built in len() method.

>>> len( ( 1, 2, 3) )
3
>>> a = ( 1, 2, 3, 4 )
>>> len( a )
4

[edit] Conversions

Convert list to tuples using the built in tuple() method.

>>> l = [4, 5, 6]
>>> tuple(l)
(4, 5, 6)

[edit] Uses of Tuples

Tuples can be used like lists and are appropriate when a list may be used but the size is known and small. One very useful situation is returning multiple values from a function. To return multiple values in many other languages requires creating an object or container of some type, but in Python it is easy:

def func(x,y):
    # code to compute a and b
    return (a,b)

This can be combined with the unpacking technique above in later code to retrieve both return values:

(a,b) = func(1,2)