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Dictionaries and Structuring Data

107

u birthdays = {'Alice': 'Apr 1', 'Bob': 'Dec 12', 'Carol': 'Mar 4'}

while True:

print('Enter a name: (blank to quit)')

name = input()

if name == '':

break

v if name in birthdays:

w print(birthdays[name] + ' is the birthday of ' + name)

else:

print('I do not have birthday information for ' + name)

print('What is their birthday?')

bday = input()

x birthdays[name] = bday

print('Birthday database updated.')

You create an initial dictionary and store it in

birthdays

u. You can see

if the entered name exists as a key in the dictionary with the

keyword v,

just as you did for lists. If the name is in the dictionary, you access the asso-

ciated value using square brackets w; if not, you can add it using the same

square bracket syntax combined with the assignment operator x.

When you run this program, it will look like this:

Enter a name: (blank to quit)

Alice

Apr 1 is the birthday of Alice

Enter a name: (blank to quit)

Eve

I do not have birthday information for Eve

What is their birthday?

Dec 5

Birthday database updated.

Enter a name: (blank to quit)

Eve

Dec 5 is the birthday of Eve

Enter a name: (blank to quit)

Of course, all the data you enter in this program is forgotten when the

program terminates. You’ll learn how to save data to ﬁles on the hard drive

in Chapter 8.

The keys(), values(), and items() Methods

There are three dictionary methods that will return list-like values of the

dictionary’s keys, values, or both keys and values:

keys()

values()

, and

items()

The values returned by these methods are not true lists: They cannot be

modiﬁed and do not have an

append()

method. But these data types (

dict_keys

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108

Chapter 5

dict_values

, and

dict_items

, respectively) can be used in

for

loops. To see

how these methods work, enter the following into the interactive shell:

>>> spam = {'color': 'red', 'age': 42}

>>> for v in spam.values():

print(v)

red

Here, a

for

loop iterates over each of the values in the

spam

dictionary.

for

loop can also iterate over the keys or both keys and values:

>>> for k in spam.keys():

print(k)

color

age

>>> for i in spam.items():

print(i)

('color', 'red')

('age', 42)

Using the

keys()

values()

, and

items()

methods, a

for

loop can iterate

over the keys, values, or key-value pairs in a dictionary, respectively. Notice

that the values in the

dict_items

value returned by the

items()

method are

tuples of the key and value.

If you want a true list from one of these methods, pass its list-like return

value to the

list()

function. Enter the following into the interactive shell:

>>> spam = {'color': 'red', 'age': 42}

>>> spam.keys()

dict_keys(['color', 'age'])

>>> list(spam.keys())

['color', 'age']

The

list(spam.keys())

line takes the

dict_keys

value returned from

keys()

and passes it to

list()

, which then returns a list value of

['color', 'age']

You can also use the multiple assignment trick in a

for

loop to assign

the key and value to separate variables. Enter the following into the inter-

active shell:

>>> spam = {'color': 'red', 'age': 42}

>>> for k, v in spam.items():

print('Key: ' + k + ' Value: ' + str(v))

Key: age Value: 42

Key: color Value: red

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Dictionaries and Structuring Data

109

Checking Whether a Key or Value Exists in a Dictionary

Recall from the previous chapter that the

and

not in

operators can check

whether a value exists in a list. You can also use these operators to see whether

a certain key or value exists in a dictionary. Enter the following into the

interactive shell:

>>> spam = {'name': 'Zophie', 'age': 7}

>>> 'name' in spam.keys()

True

>>> 'Zophie' in spam.values()

True

>>> 'color' in spam.keys()

False

>>> 'color' not in spam.keys()

True

>>> 'color' in spam

False

In the previous example, notice that

'color' in spam

is essentially a

shorter version of writing

'color' in spam.keys()

. This is always the case: If

you ever want to check whether a value is (or isn’t) a key in the dictionary, you

can simply use the

(or

not in

) keyword with the dictionary value itself.

The get() Method

It’s tedious to check whether a key exists in a dictionary before accessing

that key’s value. Fortunately, dictionaries have a

get()

method that takes two

arguments: the key of the value to retrieve and a fallback value to return if

that key does not exist.

Enter the following into the interactive shell:

>>> picnicItems = {'apples': 5, 'cups': 2}

>>> 'I am bringing ' + str(picnicItems.get('cups', 0)) + ' cups.'

'I am bringing 2 cups.'

>>> 'I am bringing ' + str(picnicItems.get('eggs', 0)) + ' eggs.'

'I am bringing 0 eggs.'

Because there is no

'eggs'

key in the

picnicItems

dictionary, the default

value

is returned by the

get()

method. Without using

get()

, the code

would have caused an error message, such as in the following example:

>>> picnicItems = {'apples': 5, 'cups': 2}

>>> 'I am bringing ' + str(picnicItems['eggs']) + ' eggs.'

Traceback (most recent call last):

File "<pyshell#34>", line 1, in <module>

'I am bringing ' + str(picnicItems['eggs']) + ' eggs.'

KeyError: 'eggs'

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110

Chapter 5

The setdefault() Method

You’ll often have to set a value in a dictionary for a certain key only if that

key does not already have a value. The code looks something like this:

spam = {'name': 'Pooka', 'age': 5}

if 'color' not in spam:

spam['color'] = 'black'

The

setdefault()

method offers a way to do this in one line of code. The

ﬁrst argument passed to the method is the key to check for, and the second

argument is the value to set at that key if the key does not exist. If the key

does exist, the

setdefault()

method returns the key’s value. Enter the follow-

ing into the interactive shell:

>>> spam = {'name': 'Pooka', 'age': 5}

>>> spam.setdefault('color', 'black')

'black'

>>> spam

{'color': 'black', 'age': 5, 'name': 'Pooka'}

>>> spam.setdefault('color', 'white')

'black'

>>> spam

{'color': 'black', 'age': 5, 'name': 'Pooka'}

The ﬁrst time

setdefault()

is called, the dictionary in

spam

changes

{'color': 'black', 'age': 5, 'name': 'Pooka'}

. The method returns the

value

'black'

because this is now the value set for the key

'color'

. When

spam.setdefault('color', 'white')

is called next, the value for that key is not

changed to

'white'

because

spam

already has a key named

'color'

The

setdefault()

method is a nice shortcut to ensure that a key exists.

Here is a short program that counts the number of occurrences of each let-

ter in a string. Open the ﬁle editor window and enter the following code,

saving it as characterCount.py:

message = 'It was a bright cold day in April, and the clocks were striking thirteen.'

count = {}

for character in message:

count.setdefault(character, 0)

count[character] = count[character] + 1

print(count)

The program loops over each character in the

message

variable’s string,

counting how often each character appears. The

setdefault()

method call

ensures that the key is in the

count

dictionary (with a default value of

)

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Dictionaries and Structuring Data

111

so the program doesn’t throw a

KeyError

error when

count[character] =

count[character] + 1

is executed. When you run this program, the output

will look like this:

{' ': 13, ',': 1, '.': 1, 'A': 1, 'I': 1, 'a': 4, 'c': 3, 'b': 1, 'e': 5, 'd': 3, 'g': 2, 'i':

6, 'h': 3, 'k': 2, 'l': 3, 'o': 2, 'n': 4, 'p': 1, 's': 3, 'r': 5, 't': 6, 'w': 2, 'y': 1}

From the output, you can see that the lowercase letter c appears 3 times,

the space character appears 13 times, and the uppercase letter A appears

1 time. This program will work no matter what string is inside the

message

variable, even if the string is millions of characters long!

Pretty Printing

If you import the

pprint

module into your programs, you’ll have access to

the

pprint()

and

pformat()

functions that will “pretty print” a dictionary’s

values. This is helpful when you want a cleaner display of the items in a

dictionary than what

print()

provides. Modify the previous characterCount.py

program and save it as prettyCharacterCount.py.

import pprint

message = 'It was a bright cold day in April, and the clocks were striking

thirteen.'

count = {}

for character in message:

count.setdefault(character, 0)

count[character] = count[character] + 1

pprint.pprint(count)

This time, when the program is run, the output looks much cleaner,

with the keys sorted.

{' ': 13,

',': 1,

'.': 1,

'A': 1,

'I': 1,

'a': 4,

'b': 1,

'c': 3,

'd': 3,

'e': 5,

'g': 2,

'h': 3,

'i': 6,

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112

Chapter 5

'k': 2,

'l': 3,

'n': 4,

'o': 2,

'p': 1,

'r': 5,

's': 3,

't': 6,

'w': 2,

'y': 1}

The

pprint.pprint()

function is especially helpful when the dictionary

itself contains nested lists or dictionaries.

If you want to obtain the prettiﬁed text as a string value instead of dis-

playing it on the screen, call

pprint.pformat()

instead. These two lines are

equivalent to each other:

pprint.pprint(someDictionaryValue)

print(pprint.pformat(someDictionaryValue))

using data Structures to model real-world things

Even before the Internet, it was possible to play a game of chess with some-

one on the other side of the world. Each player would set up a chessboard at

their home and then take turns mailing a postcard to each other describing

each move. To do this, the players needed a way to unambiguously describe

the state of the board and their moves.

In algebraic chess notation, the spaces on the chessboard are identiﬁed by

a number and letter coordinate, as in Figure 5-1.

Figure 5-1: The coordinates of

a chessboard in algebraic chess

notation

The chess pieces are identiﬁed by letters: K for king, Q for queen, R for

rook, B for bishop, and N for knight. Describing a move uses the letter of the

piece and the coordinates of its destination. A pair of these moves describes

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Dictionaries and Structuring Data

113

what happens in a single turn (with white going ﬁrst); for instance, the

notation 2. Nf3 Nc6 indicates that white moved a knight to f3 and black

moved a knight to c6 on the second turn of the game.

There’s a bit more to algebraic notation than this, but the point is that

you can use it to unambiguously describe a game of chess without needing

to be in front of a chessboard. Your opponent can even be on the other side

of the world! In fact, you don’t even need a physical chess set if you have a

good memory: You can just read the mailed chess moves and update boards

you have in your imagination.

Computers have good memories. A program on a modern computer

can easily store billions of strings like

'2. Nf3 Nc6'

. This is how computers can

play chess without having a physical chessboard. They model data to repre-

sent a chessboard, and you can write code to work with this model.

This is where lists and dictionaries can come in. You can use them to

model real-world things, like chessboards. For the ﬁrst example, you’ll use

a game that’s a little simpler than chess: tic-tac-toe.

A Tic-Tac-Toe Board

A tic-tac-toe board looks like a large hash

symbol (#) with nine slots that can each

contain an X, an O, or a blank. To repre-

sent the board with a dictionary, you can

assign each slot a string-value key, as shown

in Figure 5-2.

You can use string values to represent

what’s in each slot on the board:

'X'

'O'

, or

' '

(a space character). Thus, you’ll need

to store nine strings. You can use a diction-

ary of values for this. The string value with

the key

'top-R'

can represent the top-right

corner, the string value with the key

'low-L'

can represent the bottom-left corner, the

string value with the key

'mid-M'

can repre-

sent the middle, and so on.

This dictionary is a data structure that represents a tic-tac-toe board.

Store this board-as-a-dictionary in a variable named

theBoard

. Open a

new ﬁle editor window, and enter the following source code, saving it as

ticTacToe.py:

theBoard = {'top-L': ' ', 'top-M': ' ', 'top-R': ' ',

'mid-L': ' ', 'mid-M': ' ', 'mid-R': ' ',

'low-L': ' ', 'low-M': ' ', 'low-R': ' '}

The data structure stored in the

theBoard

variable represents the tic-tac-

toe board in Figure 5-3.

'low-L'

'low-M'

'low-R'

'mid-L'

'mid-M'

'mid-R'

'top-L'

'top-M'

'top-R'

Figure 5-2: The slots of a tic-tac-

toe board with their correspond-

ing keys

114

Chapter 5

Figure 5-3: An empty tic-tac-toe board

Since the value for every key in

theBoard

is a single-space string, this

dictionary represents a completely clear board. If player X went ﬁrst and

chose the middle space, you could represent that board with this dictionary:

theBoard = {'top-L': ' ', 'top-M': ' ', 'top-R': ' ',

'mid-L': ' ', 'mid-M': 'X', 'mid-R': ' ',

'low-L': ' ', 'low-M': ' ', 'low-R': ' '}

The data structure in

theBoard

now represents the tic-tac-toe board in

Figure 5-4.

Figure 5-4: The ﬁrst move

A board where player O has won by placing Os across the top might

look like this:

theBoard = {'top-L': 'O', 'top-M': 'O', 'top-R': 'O',

'mid-L': 'X', 'mid-M': 'X', 'mid-R': ' ',

'low-L': ' ', 'low-M': ' ', 'low-R': 'X'}

The data structure in

theBoard

now represents the tic-tac-toe board in

Figure 5-5.

Dictionaries and Structuring Data

115

Figure 5-5: Player O wins.

Of course, the player sees only what is printed to the screen, not the

contents of variables. Let’s create a function to print the board dictionary

onto the screen. Make the following addition to ticTacToe.py (new code is in

bold):

theBoard = {'top-L': ' ', 'top-M': ' ', 'top-R': ' ',

'mid-L': ' ', 'mid-M': ' ', 'mid-R': ' ',

'low-L': ' ', 'low-M': ' ', 'low-R': ' '}

def printBoard(board):

print(board['top-L'] + '|' + board['top-M'] + '|' + board['top-R'])

print('-+-+-')

print(board['mid-L'] + '|' + board['mid-M'] + '|' + board['mid-R'])

print('-+-+-')

print(board['low-L'] + '|' + board['low-M'] + '|' + board['low-R'])

printBoard(theBoard)

When you run this program,

printBoard()

will print out a blank tic-tac-

toe board.

| |

-+-+-

| |

-+-+-

| |

The

printBoard()

function can handle any tic-tac-toe data structure you

pass it. Try changing the code to the following:

theBoard = {'top-L': 'O', 'top-M': 'O', 'top-R': 'O', 'mid-L': 'X', 'mid-M':

'X', 'mid-R': ' ', 'low-L': ' ', 'low-M': ' ', 'low-R': 'X'}

def printBoard(board):

print(board['top-L'] + '|' + board['top-M'] + '|' + board['top-R'])

print('-+-+-')

print(board['mid-L'] + '|' + board['mid-M'] + '|' + board['mid-R'])

print('-+-+-')

print(board['low-L'] + '|' + board['low-M'] + '|' + board['low-R'])

printBoard(theBoard)

116

Chapter 5

Now when you run this program, the new board will be printed to

the screen.

O|O|O

-+-+-

X|X|

-+-+-

| |X

Because you created a data structure to represent a tic-tac-toe board

and wrote code in

printBoard()

to interpret that data structure, you now

have a program that “models” the tic-tac-toe board. You could have orga-

nized your data structure differently (for example, using keys like

'TOP-LEFT'

instead of

'top-L'

), but as long as the code works with your data structures,

you will have a correctly working program.

For example, the

printBoard()

function expects the tic-tac-toe data struc-

ture to be a dictionary with keys for all nine slots. If the dictionary you passed

was missing, say, the

'mid-L'

key, your program would no longer work.

O|O|O

-+-+-

Traceback (most recent call last):

File "ticTacToe.py", line 10, in <module>

printBoard(theBoard)

File "ticTacToe.py", line 6, in printBoard

print(board['mid-L'] + '|' + board['mid-M'] + '|' + board['mid-R'])

KeyError: 'mid-L'

Now let’s add code that allows the players to enter their moves. Modify

the ticTacToe.py program to look like this:

theBoard = {'top-L': ' ', 'top-M': ' ', 'top-R': ' ', 'mid-L': ' ', 'mid-M': '

', 'mid-R': ' ', 'low-L': ' ', 'low-M': ' ', 'low-R': ' '}

def printBoard(board):

print(board['top-L'] + '|' + board['top-M'] + '|' + board['top-R'])

print('-+-+-')

print(board['mid-L'] + '|' + board['mid-M'] + '|' + board['mid-R'])

print('-+-+-')

print(board['low-L'] + '|' + board['low-M'] + '|' + board['low-R'])

turn = 'X'

for i in range(9):

u printBoard(theBoard)

print('Turn for ' + turn + '. Move on which space?')

v move = input()

w theBoard[move] = turn

x if turn == 'X':

turn = 'O'

else:

turn = 'X'

printBoard(theBoard)

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