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<!DOCTYPE html>

<html>

<head>

<title>Tuples</title>

</head>

<body>

<hr />

<h1 class="chapter" id="sec125">Chapter&#XA0;10&#XA0;&#XA0;Tuples</h1>

<h2 class="section" id="sec126">10.1&#XA0;&#XA0;Tuples are immutable</h2>

<a id="hevea_default639"></a>A tuple<a id="text11" href="#note11">1</a>

is a sequence of values much like a list.

The values stored in a tuple can be any type, and

they are indexed by integers.

The important difference is that tuples are immutable.

Tuples are also comparable and hashable so we can

sort lists of them and use tuples as key values in Python

dictionaries.<a id="hevea_default640"></a>

<a id="hevea_default643"></a>Syntactically, a tuple is a comma-separated list of values:<pre class="verbatim">>>> t = 'a', 'b', 'c', 'd', 'e'

</pre>Although it is not necessary, it is common to enclose tuples in

parentheses to help us quickly identify tuples when we look at

Python code:<a id="hevea_default644"></a><pre class="verbatim">>>> t = ('a', 'b', 'c', 'd', 'e')

</pre>To create a tuple with a single element, you have to include the final

comma:<a id="hevea_default645"></a>

<a id="hevea_default646"></a><pre class="verbatim">>>> t1 = ('a',)

>>> type(t1)

</pre>Without the comma Python treats <code>('a')</code> as an expression

with a string in parentheses that evaluates to a string:<pre class="verbatim">>>> t2 = ('a')

>>> type(t2)

</pre>Another way to construct a tuple is the built-in function tuple.

With no argument, it creates an empty tuple:<a id="hevea_default647"></a>

<a id="hevea_default648"></a><pre class="verbatim">>>> t = tuple()

>>> print t

()

</pre>If the argument is a sequence (string, list, or tuple), the result

of the call to tuple is a tuple with the elements of the sequence:<pre class="verbatim">>>> t = tuple('lupins')

>>> print t

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

</pre>Because tuple is the name of a constructor, you should

avoid using it as a variable name.Most list operators also work on tuples. The bracket operator

indexes an element:<a id="hevea_default649"></a>

<a id="hevea_default650"></a><pre class="verbatim">>>> t = ('a', 'b', 'c', 'd', 'e')

>>> print t[0]

'a'

</pre>And the slice operator selects a range of elements.<a id="hevea_default651"></a>

<a id="hevea_default654"></a><pre class="verbatim">>>> print t[1:3]

('b', 'c')

</pre>But if you try to modify one of the elements of the tuple, you get

an error:<a id="hevea_default655"></a>

<a id="hevea_default658"></a><pre class="verbatim">>>> t[0] = 'A'

TypeError: object doesn't support item assignment

</pre>You can&#X2019;t modify the elements of a tuple, but you can replace

one tuple with another:<pre class="verbatim">>>> t = ('A',) + t[1:]

>>> print t

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

</pre>

<h2 class="section" id="sec127">10.2&#XA0;&#XA0;Comparing tuples</h2>

<a id="hevea_default662"></a>The comparison 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 element,

and so on, until it finds elements that differ. Subsequent

elements are not considered (even if they are really big).<pre class="verbatim">>>> (0, 1, 2) < (0, 3, 4)

True

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

True

</pre>The sort function works the same way. It sorts

primarily by first element, but in the case of a tie, it sorts

by second element, and so on. This feature lends itself to a pattern called DSU for <dl class="description"><dt class="dt-description">Decorate</dt><dd class="dd-description"> a sequence by building a list of tuples

with one or more sort keys preceding the elements from the sequence,</dd><dt class="dt-description">Sort</dt><dd class="dd-description"> the list of tuples using the Python built-in sort, and</dd><dt class="dt-description">Undecorate</dt><dd class="dd-description"> by extracting the sorted elements of the sequence.</dd></dl><a id="DSU"></a>

<a id="hevea_default667"></a>For example, suppose you have a list of words and you want to

sort them from longest to shortest:<pre class="verbatim">txt = 'but soft what light in yonder window breaks'

words = txt.split()

t = list()

for word in words:

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

t.sort(reverse=True)

res = list()

for length, word in t:

res.append(word)

print res

</pre>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.<a id="hevea_default668"></a>

<a id="hevea_default670"></a>The second loop traverses the list of tuples and builds a list of

words in descending order of length. The four-character words

are sorted in reverse alphabetical order, so &#X201C;what&#X201D; appears

before &#X201C;soft&#X201D; in the following list.The output of the program is as follows:

<pre class="verbatim">['yonder', 'window', 'breaks', 'light', 'what',

'soft', 'but', 'in']

</pre>Of course the line loses much of its poetic impact

when turned into a Python list and sorted in

descending word length order.

<h2 class="section" id="sec128">10.3&#XA0;&#XA0;Tuple assignment</h2>

<a id="hevea_default674"></a>One of the unique syntactic features of the Python language

is the ability to have a tuple on the left

side of an assignment statement. This allows you to assign

more than one variable at a time when the left side is a

sequence.In this example we have a two-element list (which is a sequence) and

assign the first and second elements of the sequence

to the variables x and y in a single statement.<pre class="verbatim">>>> m = [ 'have', 'fun' ]

>>> x, y = m

>>> x

'have'

>>> y

'fun'

>>>

</pre>It is not magic, Python roughly translates the

tuple assignment syntax

to be the following:<a id="text12" href="#note12">2</a><pre class="verbatim">>>> m = [ 'have', 'fun' ]

>>> x = m[0]

>>> y = m[1]

>>> x

'have'

>>> y

'fun'

>>>

</pre>Stylistically when we use a tuple on the left side of the assignment

statement, we omit the parentheses, but the following is an equally

valid syntax:<pre class="verbatim">>>> m = [ 'have', 'fun' ]

>>> (x, y) = m

>>> x

'have'

>>> y

'fun'

>>>

</pre>A particularly clever application of tuple assignment allows

us to swap the values of two variables in a single statement:<pre class="verbatim">>>> a, b = b, a

</pre>Both sides of this statement are tuples, but

the left side is a tuple of variables; the right side is a tuple of

expressions. Each value on the right side

is assigned to its respective variable on the left side.

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 must be the same:<a id="hevea_default675"></a>

<a id="hevea_default676"></a><pre class="verbatim">>>> a, b = 1, 2, 3

ValueError: too many values to unpack

</pre>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:<a id="hevea_default677"></a>

<a id="hevea_default679"></a><pre class="verbatim">>>> addr = 'monty@python.org'

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

</pre>The return value from split is a list with two elements;

the first element is assigned to uname, the second to

domain.<pre class="verbatim">>>> print uname

monty

>>> print domain

python.org

</pre>

<h2 class="section" id="sec129">10.4&#XA0;&#XA0;Dictionaries and tuples</h2>

<a id="hevea_default683"></a>Dictionaries have a method called items that returns a list of

tuples, where each tuple is a key-value

pair<a id="text13" href="#note13">3</a>.<pre class="verbatim">>>> d = {'a':10, 'b':1, 'c':22}

>>> t = d.items()

>>> print t

[('a', 10), ('c', 22), ('b', 1)]

</pre>As you should expect from a dictionary, the items are in no

particular order.However, since the list of tuples is a list, and tuples are comparable,

we can now sort the list of tuples. Converting a dictionary

to a list of tuples is a way for us to output the contents of a

dictionary sorted by key:<pre class="verbatim">>>> 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)]

</pre>The new list is sorted in ascending alphabetical order by the key value.

<h2 class="section" id="sec130">10.5&#XA0;&#XA0;Multiple assignment with dictionaries</h2>

<a id="hevea_default685"></a>Combining items, tuple assignment, and for, you

can see a nice code pattern for traversing the keys and values of a dictionary

in a single loop:<pre class="verbatim">for key, val in d.items():

print val, key

</pre>This loop has two iteration variables because items returns

a list of tuples and key, val is a tuple assignment

that successively iterates through each of the key-value pairs in the

dictionary. For each iteration

through the loop, both key and value are advanced to the

next key-value pair in the dictionary (still in hash order).The output of this loop is:<pre class="verbatim">10 a

22 c

1 b

</pre>Again, it is in hash key order (i.e., no particular order).If we combine these two techniques, we can print out the contents

of a dictionary sorted by the value stored in each key-value

pair.To do this, we first make a list of tuples where each tuple is

(value, key). The items method would give us a list of

(key, value) tuples&#X2014;but this time we want to sort by value, not key.

Once we have constructed the list with the value-key tuples, it is a simple

matter to sort the list in reverse order and print out the new, sorted list.<pre class="verbatim">>>> d = {'a':10, 'b':1, 'c':22}

>>> l = list()

>>> for key, val in d.items() :

... l.append( (val, key) )

...

>>> l

[(10, 'a'), (22, 'c'), (1, 'b')]

>>> l.sort(reverse=True)

>>> l

[(22, 'c'), (10, 'a'), (1, 'b')]

>>>

</pre>By carefully constructing the list of tuples to have the value as the first

element of each tuple, we can sort the list of tuples and get our dictionary

contents sorted by value.

<h2 class="section" id="sec131">10.6&#XA0;&#XA0;The most common words</h2>

Coming back to our running example of the text from Romeo and Juliet

Act 2, Scene 2, we can augment our program to use this technique to

print the ten most common words in the text as follows:<pre class="verbatim">import string

fhand = open('romeo-full.txt')

counts = dict()

for line in fhand:

line = line.translate(None, string.punctuation)

line = line.lower()

words = line.split()

for word in words:

if word not in counts:

counts[word] = 1

else:

counts[word] += 1

# Sort the dictionary by value

lst = list()

for key, val in counts.items():

lst.append( (val, key) )

lst.sort(reverse=True)

for key, val in lst[:10] :

print key, val

</pre>The first part of the program which reads the file and computes

the dictionary that maps each word to the count of words in the

document is unchanged. But instead of simply printing out

counts and ending the program, we construct a list

of (val, key) tuples and then sort the list in reverse order.Since the value is first, it will be used for the comparisons.

If there is more than one tuple with the same value, it will look

at the second element (the key), so tuples where the value is the

same will be further sorted by the alphabetical order of the key.At the end we write a nice for loop which does a multiple

assignment iteration and prints out the ten most common words

by iterating through a slice of the list (lst[:10]).So now the output finally looks like what we want for our word

frequency analysis.<pre class="verbatim">61 i

42 and

40 romeo

34 to

34 the

32 thou

32 juliet

30 that

29 my

24 thee

</pre>The fact that this complex data parsing and analysis

can be done with an easy-to-understand 19-line Python

program is one reason why Python is a good choice as a language

for exploring information.

<h2 class="section" id="sec132">10.7&#XA0;&#XA0;Using tuples as keys in dictionaries</h2>

<a id="hevea_default688"></a>Because tuples are hashable and lists are not, if we want to

create a composite key to use in a dictionary we must use a tuple as

the key.We would encounter a composite key if we wanted to create a

telephone directory that maps

from last-name, first-name pairs to telephone numbers. Assuming

that we have defined the variables

last, first, and number, we could write

a dictionary assignment statement as follows:<pre class="verbatim">directory[last,first] = number

</pre>The expression in brackets is a tuple. We could use tuple

assignment in a for loop to traverse this dictionary.<a id="hevea_default689"></a><pre class="verbatim">for last, first in directory:

print first, last, directory[last,first]

</pre>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.

<h2 class="section" id="sec133">10.8&#XA0;&#XA0;Sequences: strings, lists, and tuples&#X2014;Oh My!</h2>

<a id="hevea_default690"></a>I have focused on lists of tuples, but almost all of the examples in

this chapter also work with lists of lists, tuples of tuples, and

tuples of lists. To avoid enumerating the possible combinations, it

is sometimes easier to talk about sequences of sequences.In many contexts, the different kinds of sequences (strings, lists, and

tuples) can be used interchangeably. So how and why do you choose one

over the others?<a id="hevea_default691"></a>

<a id="hevea_default695"></a>To start with the obvious, strings are more limited than other

sequences because the elements have to be characters. They are

also immutable. If you need the ability to change the characters

in a string (as opposed to creating a new string), you might

want to use a list of characters instead.Lists are more common than tuples, mostly because they are mutable.

But there are a few cases where you might prefer tuples:<ol class="enumerate" type="1"><li class="li-enumerate">In some contexts, like a return statement, it is

syntactically simpler to create a tuple than a list. In other

contexts, you might prefer a list.</li><li class="li-enumerate">If you want to use a sequence as a dictionary key, you

have to use an immutable type like a tuple or string.</li><li class="li-enumerate">If you are passing a sequence as an argument to a function,

using tuples reduces the potential for unexpected behavior

due to aliasing.</li></ol>Because tuples are immutable, they don&#X2019;t provide methods

like sort and reverse, which modify existing lists.

However Python provides the built-in functions sorted

and reversed, which take any sequence as a parameter

and return a new sequence with the same elements in a different

order.<a id="hevea_default696"></a>

<h2 class="section" id="sec134">10.9&#XA0;&#XA0;Debugging</h2>

<a id="hevea_default703"></a>Lists, dictionaries and tuples are known generically as data

structures; in this chapter we are starting to see compound data

structures, like lists of tuples, and dictionaries that contain tuples

as keys and lists as values. Compound data structures are useful, but

they are prone to what I call shape errors; that is, errors

caused when a data structure has the wrong type, size, or composition,

or perhaps you write some code and forget the shape of your data

and introduce an error.For example, if you are expecting a list with one integer and I

give you a plain old integer (not in a list), it won&#X2019;t work.When you are debugging a program, and especially if you are

working on a hard bug, there are four things to try:<dl class="description"><dt class="dt-description">reading:</dt><dd class="dd-description"> Examine your code, read it back to yourself, and

check that it says what you meant to say.</dd><dt class="dt-description">running:</dt><dd class="dd-description"> Experiment by making changes and running different

versions. Often if you display the right thing at the right place

in the program, the problem becomes obvious, but sometimes you have to

spend some time to build scaffolding.</dd><dt class="dt-description">ruminating:</dt><dd class="dd-description"> Take some time to think! What kind of error

is it: syntax, runtime, semantic? What information can you get from

the error messages, or from the output of the program? What kind of

error could cause the problem you&#X2019;re seeing? What did you change

last, before the problem appeared?</dd><dt class="dt-description">retreating:</dt><dd class="dd-description"> At some point, the best thing to do is back

off, undoing recent changes, until you get back to a program that

works and that you understand. Then you can start rebuilding.</dd></dl>Beginning programmers sometimes get stuck on one of these activities

and forget the others. Each activity comes with its own failure

mode.<a id="hevea_default704"></a>For example, reading your code might help if the problem is a

typographical error, but not if the problem is a conceptual

misunderstanding. If you don&#X2019;t understand what your program does, you

can read it 100 times and never see the error, because the error is in

your head.<a id="hevea_default705"></a>Running experiments can help, especially if you run small, simple

tests. But if you run experiments without thinking or reading your

code, you might fall into a pattern I call &#X201C;random walk programming&#X201D;,

which is the process of making random changes until the program

does the right thing. Needless to say, random walk programming

can take a long time.<a id="hevea_default706"></a>

<a id="hevea_default707"></a>You have to take time to think. Debugging is like an

experimental science. You should have at least one hypothesis about

what the problem is. If there are two or more possibilities, try to

think of a test that would eliminate one of them.Taking a break helps with the thinking. So does talking.

If you explain the problem to someone else (or even to yourself), you

will sometimes find the answer before you finish asking the question.But even the best debugging techniques will fail if there are too many

errors, or if the code you are trying to fix is too big and

complicated. Sometimes the best option is to retreat, simplifying the

program until you get to something that works and that you

understand.Beginning programmers are often reluctant to retreat because

they can&#X2019;t stand to delete a line of code (even if it&#X2019;s wrong).

If it makes you feel better, copy your program into another file

before you start stripping it down. Then you can paste the pieces

back in a little bit at a time.Finding a hard bug requires reading, running, ruminating, and

sometimes retreating. If you get stuck on one of these activities,

try the others.

<h2 class="section" id="sec135">10.10&#XA0;&#XA0;Glossary</h2>

<dl class="description"><dt class="dt-description">comparable:</dt><dd class="dd-description"> A type where one value can be checked to see if it is

greater than, less than, or equal to another value of the same type.

Types which are comparable can be put in a list and sorted.

<a id="hevea_default708"></a></dd><dt class="dt-description">data structure:</dt><dd class="dd-description"> A collection of related values, often

organized in lists, dictionaries, tuples, etc.

<a id="hevea_default709"></a></dd><dt class="dt-description">DSU:</dt><dd class="dd-description"> Abbreviation of &#X201C;decorate-sort-undecorate&#X201D;, a

pattern that involves building a list of tuples, sorting, and

extracting part of the result.

<a id="hevea_default710"></a></dd><dt class="dt-description">gather:</dt><dd class="dd-description"> The operation of assembling a variable-length

argument tuple.

<a id="hevea_default711"></a></dd><dt class="dt-description">hashable:</dt><dd class="dd-description"> A type that has a hash function. Immutable

types like integers,

floats, and strings are hashable; mutable types like lists and

dictionaries are not.

<a id="hevea_default712"></a></dd><dt class="dt-description">scatter:</dt><dd class="dd-description"> The operation of treating a sequence as a list of

arguments.

<a id="hevea_default713"></a></dd><dt class="dt-description">shape (of a data structure):</dt><dd class="dd-description"> A summary of the type,

size, and composition of a data structure.

<a id="hevea_default714"></a></dd><dt class="dt-description">singleton:</dt><dd class="dd-description"> A list (or other sequence) with a single element.

<a id="hevea_default715"></a></dd><dt class="dt-description">tuple:</dt><dd class="dd-description"> An immutable sequence of elements.

<a id="hevea_default716"></a></dd><dt class="dt-description">tuple assignment:</dt><dd class="dd-description"> An assignment with a sequence on the

right side and a tuple of variables on the left. The right

side is evaluated and then its elements are assigned to the

variables on the left.

<h2 class="section" id="sec136">10.11&#XA0;&#XA0;Exercises</h2>

<div class="theorem">Exercise&#XA0;1&#XA0;&#XA0;

Revise a previous program as follows: Read and

parse the &#X201C;From&#X201D; lines and pull out the

addresses from the line. Count the number of

messages from each person using a dictionary.After all the data has been read, print

the person with the most commits by creating

a list of (count, email) tuples from the

dictionary. Then sort the list in reverse

order and print out the person who has the most

commits.<pre class="verbatim">Sample Line:

From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

Enter a file name: mbox-short.txt

cwen@iupui.edu 5

Enter a file name: mbox.txt

zqian@umich.edu 195

</pre></div><div class="theorem">Exercise&#XA0;2&#XA0;&#XA0;

This program counts the distribution of the hour of the day for

each of the messages. You can pull the hour from the &#X201C;From&#X201D;

line by finding the time string and then splitting that string

into parts using the colon character. Once you have accumulated

the counts for each hour, print out the counts, one per line,

sorted by hour as shown below.

<pre class="verbatim">Sample Execution:

python timeofday.py

Enter a file name: mbox-short.txt

04 3

06 1

07 1

09 2

10 3

11 6

14 1

15 2

16 4

17 2

18 1

19 1

</pre></div><div class="theorem">Exercise&#XA0;3&#XA0;&#XA0;

Write a program that reads a file and

prints the letters in decreasing order of frequency. Your program

should convert all the input to lower case and only count the letters a-z.

Your program should not count spaces, digits, punctuation, or anything

other than the letters a-z.

Find text samples from several different languages and see how letter frequency

varies between languages. Compare your results with the tables at

wikipedia.org/wiki/Letter_frequencies.<a id="hevea_default719"></a>

<a id="note11" href="#text11">1</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">Fun fact: The word &#X201C;tuple&#X201D; comes from the names

given to sequences of numbers of varying lengths: single,

double, triple, quadruple, quituple, sextuple, septuple, etc.</div>

</dd><dt class="dt-thefootnotes"><a id="note12" href="#text12">2</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">Python does not translate the

syntax literally. For example, if you try this with a dictionary,

it will not work as might expect.</div>

</dd><dt class="dt-thefootnotes"><a id="note13" href="#text13">3</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">This behavior is slightly different in Python 3.0.</div>

</dd></dl>

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