Chapter: Problem Solving and Python Programming : Lists, Tuples, Dictionaries

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Tuples - Python

A tuple is a sequence of values. The values can be any type, and they are indexed by integers, so in that respect tuples are a lot like lists.

TUPLES

 

Tuples are immutable

 

A tuple is a sequence of values. The values can be any type, and they are indexed by integers, so in that respect tuples are a lot like lists. The important difference is that tuples are immutable. Syntactically, a tuple is a comma-separated list of values:

>>> t = 'a', 'b', 'c', 'd', 'e'

 

Although it is not necessary, it is common to enclose tuples in parentheses:

>>> t = ('a', 'b', 'c', 'd', 'e')

 

To create a tuple with a single element, you have to include a final comma:

>>> t1 = 'a',

>>> type(t1) <type 'tuple'>

 

A value in parentheses is not a tuple:

>>>        t2 = ('a')

>>>        type(t2)

<type 'str'>

 

Another way to create a tuple is the built-in function tuple. With no argument, it creates an empty tuple:

>>> t = tuple()

>>> print t

()

 

If the argument is a sequence (string, list or tuple), the result is a tuple with the elements of the sequence:

>>> t = tuple('lupins')

>>> print t

('l', 'u', 'p', 'i', 'n', 's')

 

Because tuple is the name of a built-in function, you should avoid using it as a variable name.

Most list operators also work on tuples. The bracket operator indexes an element:

>>>        t = ('a', 'b', 'c', 'd', 'e')

>>>        print t[0]

'a'

 

And the slice operator selects a range of elements.

>>>        print t[1:3]

('b', 'c')

 

But if you try to modify one of the elements of the tuple, you get an error:

>>> t[0] = 'A'

 

TypeError: object doesn't support item assignment

 

You can’t modify the elements of a tuple, but you can replace one tuple with another:

>>>        t = ('A',) + t[1:]

>>>        print t

('A', 'b', 'c', 'd', 'e')

 

Tuple assignment

 

It is often useful to swap the values of two variables. With conventional assignments, you have to use a temporary variable. For example, to swap a and b:

>>>        temp = a

>>>        a = b

>>>        b = temp

 

This solution is cumbersome; tuple assignment is more elegant:

>>> a, b = b, a

 

The left side is a tuple of variables; the right side is a tuple of expressions. Each value is assigned to its respective variable. All the expressions on the right side are evaluated before any of the assignments.

The number of variables on the left and the number of values on the right have to be the same:

>>> a, b = 1, 2, 3

ValueError: too many values to unpack

 

More generally, the right side can be any kind of sequence (string, list or tuple). For example, to split an email address into a user name and a domain, you could write:

>>>        addr = 'monty@python.org'

>>>        uname, domain = addr.split('@')

 

The return value from split is a list with two elements; the first element is assigned to uname, the second to domain.

>>>        print uname monty

>>>        print domain python.org

 

Tuples as return values

 

A function can only return one value, but if the value is a tuple, the effect is the same as returning multiple values. For example, if you want to divide two integers and compute the quotient and remainder, it is inefficient to compute x/y and then x%y. It is better to compute them both at the same time.

 

The built-in function divmod takes two arguments and returns a tuple of two values, the quotient and remainder. You can store the result as a tuple:

>>>        t = divmod(7, 3)

>>>        print t

(2, 1)

 

Or use tuple assignment to store the elements separately:

>>>        quot, rem = divmod(7, 3)

>>>        print quot

2

>>>        print rem

1

 

Here is an example of a function that returns a tuple:

def min_max(t):

return min(t), max(t)

 

max and min are built-in functions that find the largest and smallest elements of a sequence. min_max computes both and returns a tuple of two values.

 

Variable-length argument tuples

 

Functions can take a variable number of arguments. A parameter name that begins with * gathers arguments into a tuple. For example, printall takes any number of arguments and prints them:

def printall(*args):

print args

 

The gather parameter can have any name you like, but args is conventional. Here’s howthe function works:

>>>        printall(1, 2.0, '3')

(1, 2.0, '3')

 

The complement of gather is scatter. If you have a sequence of values and you want to pass it to a function as multiple arguments, you can use the * operator. For example, divmod takes exactly two arguments; it doesn’t work with a tuple:

>>>        t = (7, 3)

>>>        divmod(t)

TypeError: divmod expected 2 arguments, got 1

 

But if you scatter the tuple, it works:

>>>        divmod(*t)

(2, 1)

>>>        max(1,2,3)

3

But sum does not.

>>>        sum(1,2,3)

TypeError: sum expected at most 2 arguments, got 3

Write a function called sumall that takes any number of arguments and returns their sum.

 

Lists and tuples

 

zip is a built-in function that takes two or more sequences and “zips” them into a list of tuples where each tuple contains one element from each sequence. In Python 3, zip returns an iterator of tuples, but for most purposes, an iterator behaves like a list.

 

This example zips a string and a list:

>>>        s = 'abc'

>>>        t = [0, 1, 2]

>>>        zip(s, t)

[('a', 0), ('b', 1), ('c', 2)]

 

The result is a list of tuples where each tuple contains a character from the string and the corresponding element from the list.

If the sequences are not the same length, the result has the length of the shorter one.

>>>        zip('Anne', 'Elk')

[('A', 'E'), ('n', 'l'), ('n', 'k')]

 

You can use tuple assignment in a for loop to traverse a list of tuples:

t = [('a', 0), ('b', 1), ('c', 2)]

for letter, number in t:

print number, letter

 

Each time through the loop, Python selects the next tuple in the list and assigns the elements to letter and number. The output of this loop is:

0 a

1 b

2 c

 

If you combine zip, for and tuple assignment, you get a useful idiom for traversing two (or more) sequences at the same time. For example, has_match takes two sequences, t1 and t2, and returns True if there is an index i such that t1[i] == t2[i]:

def has_match(t1, t2):

for x, y in zip(t1, t2):

if x == y:

return True

return False

 

If you need to traverse the elements of a sequence and their indices, you can use the built-in function enumerate:

for index, element in enumerate('abc'):

print index, element

The output of this loop is:

0 a

1 b

2 c

Again.

 

Dictionaries and tuples

 

Dictionaries have a method called items that returns a list of tuples, where each tuple is a key-value pair.

>>>        d = {'a':0, 'b':1, 'c':2}

>>>        t = d.items()

>>>        print t

[('a', 0), ('c', 2), ('b', 1)]

 

As you should expect from a dictionary, the items are in no particular order. In Python3, items returns an iterator, but for many purposes, iterators behave like lists. Going in the other direction, you can use a list of tuples to initialize a new dictionary:

>>>        t = [('a', 0), ('c', 2), ('b', 1)]

>>>        d = dict(t)

>>>        print d

{'a': 0, 'c': 2, 'b': 1}

 

Combining dict with zip yields a concise way to create a dictionary:

>>>        d = dict(zip('abc', range(3)))

>>>        print d

{'a': 0, 'c': 2, 'b': 1}

 

The dictionary method update also takes a list of tuples and adds them, as key-value pairs, to an existing dictionary. Combining items, tuple assignment and for, you get the idiom for traversing the keys and values of a dictionary:

for key, val in d.items():

print val, key

 

The output of this loop is:

0 a

2 c

1b Again.

 

It is common to use tuples as keys in dictionaries (primarily because you can’t use lists). For example, a telephone directory might map from last-name, first-name pairs to telephone numbers. Assuming that we have defined last, first and number, we could write: directory[last,first] = number

 

The expression in brackets is a tuple. We could use tuple assignment to traverse this dictionary.


for last, first in directory:

print first, last, directory[last,first]

 

This loop traverses the keys in directory, which are tuples. It assigns the elements of each tuple to last and first, then prints the name and corresponding telephone number. There are two ways to represent tuples in a state diagram. The more detailed version shows the indices and elements just as they appear in a list.

 

Comparing tuples

 

The relational operators work with tuples and other sequences; Python starts by comparing the first element from each sequence. If they are equal, it goes on to the next elements, and so on, until it finds elements that differ. Subsequent elements are not considered (even if they are really big).

 

>>>        (0, 1, 2) < (0, 3, 4)

True

>>>        (0, 1, 2000000) < (0, 3, 4)

True

 

The sort function works the same way. It sorts primarily by first element, but in the caseof a tie, it sorts by second element, and so on.

 

This feature lends itself to a pattern called DSU for Decorate a sequence by building a list of tuples with one or more sort keys preceding the elements from the sequence,

Sort the list of tuples, and 

Undecorate by extracting the sorted elements of the sequence.

For example, suppose you have a list of words and you want to sort them from longest to shortest:

def sort_by_length(words):

t = []

for word in words:

t.append((len(word), word))

t.sort(reverse=True)

res = []

for length, word in t:

res.append(word)

return res

 

The first loop builds a list of tuples, where each tuple is a word preceded by its length. sort compares the first element, length, first, and only considers the second element to break ties. The keyword argument reverse=True tells sort to go in decreasing order.

 

The second loop traverses the list of tuples and builds a list of words in descending order of length.

 

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