Python Programming/Tuples

From Wikibooks, open books for an open world
Jump to: navigation, search

A tuple in Python is much like a list except that it is immutable (unchangeable) once created, and hashable and thus suitable as a key in a dictionary.

Tuple notation[edit]

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

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

A one item tuple is created by an item in parentheses 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')

Packing and Unpacking[edit]

You can also perform multiple assignment using tuples.

>>> article, noun, verb, adjective, direct_object = t   #t is defined above
>>> noun
'tuple'

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

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

The example above: article, noun, verb, adjective, direct_object = t is called "tuple unpacking" because the tuple t was unpacked and its values assigned to each of the variables on the left. "Tuple packing" is the reverse: t=article, noun, verb, adjective, direct_object. When unpacking a tuple, or performing multiple assignment, you must have the same number of variables being assigned to as values being assigned.

Operations on tuples[edit]

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)
>>> 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]

Tuple Attributes[edit]

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

Conversions[edit]

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

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

Converting a tuple into a list using the built in list() method to cast as a list:

>>> t = (4, 5, 6)
>>> list(t)
[4, 5, 6]

Dictionaries can also be converted to tuples of tuples using the items method of dictionaries:

>>> d = {'a': 1, 'b': 2}
>>> tuple(d.items())
(('a', 1), ('b', 2))

Uses of Tuples[edit]

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 x and y
    return (x,y)

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

(x,y) = func(1,2)

Using List Comprehension to process Tuple elements[edit]

Occasionally, there is a need to manipulate the values contained within a tuple in order to create a new tuple. For example, if we wanted a way to double all of the values within a tuple, we can combine some of the above information in addition to list comprehension like this:

def double(T):
    'double() - return a tuple with each tuple element (e) doubled.'
    return tuple( [ e * 2 for e in T ] )

Exercises[edit]

  1. Create the list ['a', 'b', 'c'], then create a tuple from that list.
  2. Create the tuple ('a', 'b', 'c'), then create a list from that tuple. (Hint: the material needed to do this has been covered, but it's not entirely obvious)
  3. Make the following instantiations simultaneously: a = 'a', b=2, c='gamma'. (That is, in one line of code).
  4. Create a tuple containing just a single element which in turn contains the three elements 'a', 'b', and 'c'. Verify that the length is actually 1 by using the len() function.

External links[edit]