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Flow Control

importing modules

All Python programs can call a basic set of functions called built-in functions,

including the

print()

input()

, and

len()

functions you’ve seen before. Python

also comes with a set of modules called the standard library. Each module

is a Python program that contains a related group of functions that can be

embedded in your programs. For example, the

math

module has mathematics-

related functions, the

random

module has random number–related functions,

and so on.

Before you can use the functions in a module, you must import the

module with an

import

statement. In code, an

import

statement consists of

the following:

• The

import

keyword

• The name of the module

• Optionally, more module names, as long as they are separated by

commas

Once you import a module, you can use all the cool functions of that

module. Let’s give it a try with the

random

module, which will give us access

to the

random.ranint()

function.

Enter this code into the ﬁle editor, and save it as printRandom.py:

import random

for i in range(5):

print(random.randint(1, 10))

When you run this program, the output will look something like this:

The

random.randint()

function call evaluates to a random integer value

between the two integers that you pass it. Since

randint()

is in the

random

module, you must ﬁrst type

random.

in front of the function name to tell

Python to look for this function inside the

random

module.

Here’s an example of an

import

statement that imports four different

modules:

import random, sys, os, math

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Chapter 2

Now we can use any of the functions in these four modules. We’ll learn

more about them later in the book.

from import Statements

An alternative form of the

import

statement is composed of the

from

key-

word, followed by the module name, the

import

keyword, and a star; for

example,

from random import *.

With this form of

import

statement, calls to functions in

random

will not

need the

random.

preﬁx. However, using the full name makes for more read-

able code, so it is better to use the normal form of the

import

statement.

ending a Program early with sys.exit()

The last ﬂow control concept to cover is how to terminate the program.

This always happens if the program execution reaches the bottom of the

instructions. However, you can cause the program to terminate, or exit, by

calling the

sys.exit()

function. Since this function is in the

sys

module, you

have to import

sys

before your program can use it.

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

exitExample.py:

import sys

while True:

print('Type exit to exit.')

response = input()

if response == 'exit':

sys.exit()

print('You typed ' + response + '.')

Run this program in IDLE. This program has an inﬁnite loop with no

break

statement inside. The only way this program will end is if the user enters

exit

, causing

sys.exit()

to be called. When

response

is equal to

exit

, the pro-

gram ends. Since the

response

variable is set by the

input()

function, the user

must enter

exit

in order to stop the program.

Summary

By using expressions that evaluate to

True

False

(also called conditions),

you can write programs that make decisions on what code to execute and

what code to skip. You can also execute code over and over again in a loop

while a certain condition evaluates to

True

. The

break

and

continue

statements

are useful if you need to exit a loop or jump back to the start.

These ﬂow control statements will let you write much more intelligent

programs. There’s another type of ﬂow control that you can achieve by writ-

ing your own functions, which is the topic of the next chapter.

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Flow Control

Practice Questions

1. What are the two values of the Boolean data type? How do you

write them?

2. What are the three Boolean operators?

3. Write out the truth tables of each Boolean operator (that is, every

possible combination of Boolean values for the operator and what

they evaluate to).

4. What do the following expressions evaluate to?

(5 > 4) and (3 == 5)

not (5 > 4)

(5 > 4) or (3 == 5)

not ((5 > 4) or (3 == 5))

(True and True) and (True == False)

(not False) or (not True)

5. What are the six comparison operators?

6. What is the difference between the equal to operator and the assign-

ment operator?

7. Explain what a condition is and where you would use one.

8. Identify the three blocks in this code:

spam = 0

if spam == 10:

print('eggs')

if spam > 5:

print('bacon')

else:

print('ham')

print('spam')

9. Write code that prints

Hello

is stored in

spam

, prints

Howdy

stored in

spam

, and prints

Greetings!

if anything else is stored in

spam

10. What can you press if your program is stuck in an inﬁnite loop?

11. What is the difference between

break

and

continue

12. What is the difference between

range(10)

range(0, 10)

, and

range(0, 10, 1)

in a

for

loop?

13. Write a short program that prints the numbers

using a

for

loop.

Then write an equivalent program that prints the numbers

using

while

loop.

14. If you had a function named

bacon()

inside a module named

spam

, how

would you call it after importing

spam

Extra credit: Look up the

round()

and

abs()

functions on the Internet,

and ﬁnd out what they do. Experiment with them in the interactive shell.

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fu n C t i o n S

You’re already familiar with the

print()

input()

, and

len()

functions from the previ-

ous chapters. Python provides several built-

in functions like these, but you can also write

your own functions. A function is like a mini- program

within a program.

To better understand how functions work, let’s create one. Type this

program into the ﬁle editor and save it as helloFunc.py:

u def hello():

v print('Howdy!')

print('Howdy!!!')

print('Hello there.')

w hello()

hello()

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Chapter 3

The ﬁrst line is a

def

statement u, which deﬁnes a function named

hello()

. The code in the block that follows the

def

statement v is the body

of the function. This code is executed when the function is called, not when

the function is ﬁrst deﬁned.

The

hello()

lines after the function w are function calls. In code, a

function call is just the function’s name followed by parentheses, possibly

with some number of arguments in between the parentheses. When the

program execution reaches these calls, it will jump to the top line in the

function and begin executing the code there. When it reaches the end of

the function, the execution returns to the line that called the function

and continues moving through the code as before.

Since this program calls

hello()

three times, the code in the

hello()

function is executed three times. When you run this program, the output

looks like this:

Howdy!

Howdy!!!

Hello there.

Howdy!

Howdy!!!

Hello there.

Howdy!

Howdy!!!

Hello there.

A major purpose of functions is to group code that gets executed mul-

tiple times. Without a function deﬁned, you would have to copy and paste

this code each time, and the program would look like this:

print('Howdy!')

print('Howdy!!!')

print('Hello there.')

print('Howdy!')

print('Howdy!!!')

print('Hello there.')

print('Howdy!')

print('Howdy!!!')

print('Hello there.')

In general, you always want to avoid duplicating code, because if you

ever decide to update the code—if, for example, you ﬁnd a bug you need to

ﬁx—you’ll have to remember to change the code everywhere you copied it.

As you get more programming experience, you’ll often ﬁnd yourself

deduplicating code, which means getting rid of duplicated or copy-and-

pasted code. Deduplication makes your programs shorter, easier to read,

and easier to update.

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Functions

def Statements with Parameters

When you call the

print()

len()

function, you pass in values, called argu-

ments in this context, by typing them between the parentheses. You can also

deﬁne your own functions that accept arguments. Type this example into

the ﬁle editor and save it as helloFunc2.py:

u def hello(name):

v print('Hello ' + name)

w hello('Alice')

hello('Bob')

When you run this program, the output looks like this:

Hello Alice

Hello Bob

The deﬁnition of the

hello()

function in this program has a parameter

called

name

u. A parameter is a variable that an argument is stored in when a

function is called. The ﬁrst time the

hello()

function is called, it’s with the

argument

'Alice'

w. The program execution enters the function, and the

variable

name

is automatically set to

'Alice'

, which is what gets printed by the

print()

statement v.

One special thing to note about parameters is that the value stored

in a parameter is forgotten when the function returns. For example, if you

added

print(name)

after

hello('Bob')

in the previous program, the program

would give you a

NameError

because there is no variable named

name

. This

variable was destroyed after the function call

hello('Bob')

had returned, so

print(name)

would refer to a

name

variable that does not exist.

This is similar to how a program’s variables are forgotten when the pro-

gram terminates. I’ll talk more about why that happens later in the chapter,

when I discuss what a function’s local scope is.

return Values and return Statements

When you call the

len()

function and pass it an argument such as

'Hello'

the function call evaluates to the integer value

, which is the length of the

string you passed it. In general, the value that a function call evaluates to is

called the return value of the function.

When creating a function using the

def

statement, you can specify what

the return value should be with a

return

statement. A

return

statement con-

sists of the following:

• The

return

keyword

• The value or expression that the function should return

Chapter 3

When an expression is used with a

return

statement, the return value

is what this expression evaluates to. For example, the following program

deﬁnes a function that returns a different string depending on what num-

ber it is passed as an argument. Type this code into the ﬁle editor and save

it as magic8Ball.py:

u import random

v def getAnswer(answerNumber):

w if answerNumber == 1:

return 'It is certain'

elif answerNumber == 2:

return 'It is decidedly so'

elif answerNumber == 3:

return 'Yes'

elif answerNumber == 4:

return 'Reply hazy try again'

elif answerNumber == 5:

return 'Ask again later'

elif answerNumber == 6:

return 'Concentrate and ask again'

elif answerNumber == 7:

return 'My reply is no'

elif answerNumber == 8:

return 'Outlook not so good'

elif answerNumber == 9:

return 'Very doubtful'

x r = random.randint(1, 9)

y fortune = getAnswer(r)

z print(fortune)

When this program starts, Python ﬁrst imports the

random

module u.

Then the

getAnswer()

function is deﬁned v. Because the function is being

deﬁned (and not called), the execution skips over the code in it. Next, the

random.randint()

function is called with two arguments,

and

x. It evalu-

ates to a random integer between

and

(including

and

themselves),

and this value is stored in a variable named

The

getAnswer()

function is called with

as the argument y. The pro-

gram execution moves to the top of the

getAnswer()

function w, and the

value

is stored in a parameter named

answerNumber

. Then, depending on

this value in

answerNumber

, the function returns one of many possible string

values. The program execution returns to the line at the bottom of the pro-

gram that originally called

getAnswer()

y. The returned string is assigned to

a variable named

fortune

, which then gets passed to a

print()

call z and is

printed to the screen.

Functions

Note that since you can pass return values as an argument to another

function call, you could shorten these three lines:

r = random.randint(1, 9)

fortune = getAnswer(r)

print(fortune)

to this single equivalent line:

print(getAnswer(random.randint(1, 9)))

Remember, expressions are composed of values and operators. A func-

tion call can be used in an expression because it evaluates to its return value.

the none Value

In Python there is a value called

None

, which represents the absence of a

value.

None

is the only value of the

NoneType

data type. (Other programming

languages might call this value

null

nil

, or

undefined

.) Just like the Boolean

True

and

False

values,

None

must be typed with a capital N.

This value-without-a-value can be helpful when you need to store some-

thing that won’t be confused for a real value in a variable. One place where

None

is used is as the return value of

print()

. The

print()

function displays

text on the screen, but it doesn’t need to return anything in the same way

len()

input()

does. But since all function calls need to evaluate to a return

value,

print()

returns

None

. To see this in action, enter the following into the

interactive shell:

>>> spam = print('Hello!')

Hello!

>>> None == spam

True

Behind the scenes, Python adds

return None

to the end of any func-

tion deﬁnition with no

return

statement. This is similar to how a

while

for

loop implicitly ends with a

continue

statement. Also, if you use a

return

state-

ment without a value (that is, just the

return

keyword by itself), then

None

returned.

keyword Arguments and print()

Most arguments are identiﬁed by their position in the function call. For

example,

random.randint(1, 10)

is different from

random.randint(10, 1)

. The

function call

random.randint(1, 10)

will return a random integer between

and

, because the ﬁrst argument is the low end of the range and the sec-

ond argument is the high end (while

random.randint(10, 1)

causes an error).

Chapter 3

However, keyword arguments are identiﬁed by the keyword put before

them in the function call. Keyword arguments are often used for optional

parameters. For example, the

print()

function has the optional parameters

end

and

sep

to specify what should be printed at the end of its arguments

and between its arguments (separating them), respectively.

If you ran the following program:

print('Hello')

print('World')

the output would look like this:

Hello

World

The two strings appear on separate lines because the

print()

function

automatically adds a newline character to the end of the string it is passed.

However, you can set the

end

keyword argument to change this to a different

string. For example, if the program were this:

print('Hello', end='')

print('World')

the output would look like this:

HelloWorld

The output is printed on a single line because there is no longer a new-

line printed after

'Hello'

. Instead, the blank string is printed. This is use-

ful if you need to disable the newline that gets added to the end of every

print()

function call.

Similarly, when you pass multiple string values to

print()

, the function

will automatically separate them with a single space. Enter the following

into the interactive shell:

>>> print('cats', 'dogs', 'mice')

cats dogs mice

But you could replace the default separating string by passing the

sep

keyword argument. Enter the following into the interactive shell:

>>> print('cats', 'dogs', 'mice', sep=',')

cats,dogs,mice

You can add keyword arguments to the functions you write as well, but

ﬁrst you’ll have to learn about the list and dictionary data types in the next

two chapters. For now, just know that some functions have optional keyword

arguments that can be speciﬁed when the function is called.

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