10-Tuples

Tuples
Chapter 10
Tuples are like lists
•
Tuples are another kind of sequence that function much like
a list - they have elements which are indexed starting at 0
>>> x = ('Glenn', 'Sally', 'Joseph')
>>> print x[2]
Joseph
>>> y = ( 1, 9, 2 )
>>> print y
(1, 9, 2)
>>> print max(y)
9
>>> for iter in y:
print iter

1
9
2
>>>
Tuples are "immutable"
•
Unlike a list, once you create a tuple, you cannot alter its
contents - similar to a string
>>> x = [9, 8, 7]
>>> x[2] = 6
>>> print x
[9, 8, 6]
>>>
>>> y = 'ABC'
>>> y[2] = 'D'
Traceback:
'str' object does
not support item
assignment
>>>
>>> z = (5, 4, 3)
>>> z[2] = 0
Traceback:
'tuple' object does
not support item
assignment
>>>
Things not to do with tuples
>>> x = (3, 2, 1)
>>> x.sort()
Traceback:
AttributeError: 'tuple' object has no attribute 'sort'
>>> x.append(5)
Traceback:
AttributeError: 'tuple' object has no attribute 'append'
>>> x.reverse()
Traceback:
AttributeError: 'tuple' object has no attribute 'reverse'
>>>
A Tale of Two Sequences
>>> l = list()
>>> dir(l)
['append', 'count', 'extend', 'index', 'insert',
'pop', 'remove', 'reverse', 'sort']
>>> t = tuple()
>>> dir(t)
['count', 'index']
What *can* tuples do?
•
Since a tuples are not mutable, it is hashable - which means
that a tuple can be used as a key in a dictionary
•
Since lists can change they can not be keys in a dictionary
>>> d = dict()
>>> d[ ('Chuck', 4) ] = 'yes'
>>> print d.get( ('Chuck', 4) )
yes
>>> d[ ['Glenn', 6] ] = 'no'
Traceback:
TypeError: unhashable type: 'list'
Tuples are more efficient
•
Since Python does not have to build tuple structures to be
modifyable, they are simpler and more efficient in terms of
memory use and performance than lists
•
So we use tuples more often as "temporary variables"
instead of lists.
Tuples are Comparable
•
The comparison operators work with tuples and other
sequences If the first item is equal, Python goes on to the
next element, and so on, until it finds elements that differ.
>>> (0, 1, 2) < (5, 1, 2)
True
>>> (0, 1, 2000000) < (0, 3, 4)
True
>>> ( 'Jones', 'Sally' ) < ('Jones', 'Fred')
False
>>> ( 'Jones', 'Sally') > ('Adams', 'Sam')
True
Tuples can be Sorted
•
You cannot sort a tuple "in place" but you can create a
sorted copy of a tuple using the built-in function sorted()
>>> tup = (3, 2, 1)
>>> put = sorted(tup)
>>> print put
[1, 2, 3]
>>> print tup
(3, 2, 1)
Tuples and
Dictionaries
•
The items() method in
dictionaries returns a
list of (key, value) tuples
>>> d = dict()
>>> d['csev'] = 2
>>> d['cwen'] = 4
>>> tups = d.items()
>>> print tups
[('csev', 2), ('cwen', 4)]
>>> for (k,v) in d.items():
print k, v

csev 2
cwen 4
Tuples and Assignment
•
We can also put a tuple on the left hand side of an
assignment statement
•
We can even omit the parenthesis
>>> for (k,v) in d.items():
print k, v

csev 2
cwen 4
>>> (x, y) = (4, 'fred')
>>> print y
fred
>>> (a, b) = (99, 98)
>>> print a
99
Dictionaries and Lists of Tuples
•
We can take advantage of the ability to sort a list of tuples to
get a sorted version of a dictionary
•
First we sort the dictionary by the key using the items()
method
>>> d = {'a':10, 'b':1, 'c':22}
>>> t = d.items()
>>> t
[('a', 10), ('c', 22), ('b', 1)]
>>> t.sort()
>>> t
[('a', 10), ('b', 1), ('c', 22)]
Using sorted()
>>> d = {'a':10, 'b':1, 'c':22}
>>> t = sorted(d.items())
>>> t
[('a', 10), ('b', 1), ('c', 22)]
>>> for k, v in sorted(d.items()):
print k, v

a 10
b 1
c 22
We can do this even
more directly using the
built-in function sorted
that takes a sequence as
a parameter and returns
a sorted list of items
Sort by values instead of key
•
If we could
construct a list of
tuples of the form
(value, key) we
could sort by value
•
We do this with a
for loop that
creates a list of
tuples
>>> c = {'a':10, 'b':1, 'c':22}
>>> tmp = list()
>>> for k, v in c.items() :
tmp.append( (v, k) )

>>> print tmp
[(10, 'a'), (22, 'c'), (1, 'b')]
>>> tmp.sort(reverse=True)
>>> print tmp
[(22, 'c'), (10, 'a'), (1, 'b')]

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