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Flow Control

The computer will evaluate the left expression ﬁrst,

and then it will evaluate the right expression. When it

knows the Boolean value for each, it will then evaluate

the whole expression down to one Boolean value. You

can think of the computer’s evaluation process for

(4 < 5) and (5 < 6)

as shown in Figure 2-2.

You can also use multiple Boolean operators in an

expression, along with the comparison operators.

>>> 2 + 2 == 4 and not 2 + 2 == 5 and 2 * 2 == 2 + 2

True

The Boolean operators have an order of operations just like the

math operators do. After any math and comparison operators evaluate,

Python evaluates the

not

operators ﬁrst, then the

and

operators, and then

the

operators.

elements of flow control

Flow control statements often start with a part called the condition, and all

are followed by a block of code called the clause. Before you learn about

Python’s speciﬁc ﬂow control statements, I’ll cover what a condition and a

block are.

Conditions

The Boolean expressions you’ve seen so far could all be considered con-

ditions, which are the same thing as expressions; condition is just a more

speciﬁc name in the context of ﬂow control statements. Conditions always

evaluate down to a Boolean value,

True

False

. A ﬂow control statement

decides what to do based on whether its condition is

True

False

, and

almost every ﬂow control statement uses a condition.

Blocks of Code

Lines of Python code can be grouped together in blocks. You can tell when a

block begins and ends from the indentation of the lines of code. There are

three rules for blocks.

1. Blocks begin when the indentation increases.

2. Blocks can contain other blocks.

3. Blocks end when the indentation decreases to zero or to a containing

block’s indentation.

(4 < 5) and (5 < 6)

True and (5 < 6)

True and True

True

Figure 2-2: The

process of evalu-

ating (4 < 5) and

(5 < 6) to True.

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Chapter 2

Blocks are easier to understand by looking at some indented code, so

let’s ﬁnd the blocks in part of a small game program, shown here:

if name == 'Mary':

u print('Hello Mary')

if password == 'swordfish':

v print('Access granted.')

else:

w print('Wrong password.')

The ﬁrst block of code u starts at the line

print('Hello Mary')

and con-

tains all the lines after it. Inside this block is another block v, which has

only a single line in it:

print('Access Granted.')

. The third block w is also

one line long:

print('Wrong password.')

Program execution

In the previous chapter’s hello.py program, Python started executing

instruc tions at the top of the program going down, one after another. The

program execution (or simply, execution) is a term for the current instruction

being executed. If you print the source code on paper and put your ﬁnger

on each line as it is executed, you can think of your ﬁnger as the program

execution.

Not all programs execute by simply going straight down, however. If you

use your ﬁnger to trace through a program with ﬂow control statements,

you’ll likely ﬁnd yourself jumping around the source code based on condi-

tions, and you’ll probably skip entire clauses.

flow control Statements

Now, let’s explore the most important piece of ﬂow control: the statements

themselves. The statements represent the diamonds you saw in the ﬂowchart

in Figure 2-1, and they are the actual decisions your programs will make.

if Statements

The most common type of ﬂow control statement is the

statement. An

statement’s clause (that is, the block following the

statement) will execute

if the statement’s condition is

True

. The clause is skipped if the condition is

False

In plain English, an

statement could be read as, “If this condition is

true, execute the code in the clause.” In Python, an

statement consists of

the following:

• The

keyword

• A condition (that is, an expression that evaluates to

True

False

)

• A colon

• Starting on the next line, an indented block of code (called the

clause)

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Flow Control

For example, let’s say you have some code that checks to see whether

someone’s name is Alice. (Pretend

name

was assigned some value earlier.)

if name == 'Alice':

print('Hi, Alice.')

All ﬂow control statements end with a colon and are followed by a

new block of code (the clause). This

statement’s clause is the block with

print('Hi, Alice.')

. Figure 2-3 shows what a ﬂowchart of this code would

look like.

print('Hi, Alice.')

Start

End

name == 'Alice'

True

False

Figure 2-3: The ﬂowchart for an if statement

else Statements

clause can optionally be followed by an

else

statement. The

else

clause

is executed only when the

statement’s condition is

False

. In plain English,

else

statement could be read as, “If this condition is true, execute this

code. Or else, execute that code.” An

else

statement doesn’t have a condi-

tion, and in code, an

else

statement always consists of the following:

• The

else

keyword

• A colon

• Starting on the next line, an indented block of code (called the

else

clause)

Returning to the Alice example, let’s look at some code that uses an

else

statement to offer a different greeting if the person’s name isn’t Alice.

if name == 'Alice':

print('Hi, Alice.')

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Chapter 2

else:

print('Hello, stranger.')

Figure 2-4 shows what a ﬂowchart of this code would look like.

print('Hi, Alice.')

Start

End

name == 'Alice'

print('Hello, stranger.')

True

False

Figure 2-4: The ﬂowchart for an else statement

elif Statements

While only one of the

else

clauses will execute, you may have a case

where you want one of many possible clauses to execute. The

elif

statement

is an “else if” statement that always follows an

or another

elif

statement.

It provides another condition that is checked only if any of the previous con-

ditions were

False

. In code, an

elif

statement always consists of the following:

• The

elif

keyword

• A condition (that is, an expression that evaluates to

True

False

)

• A colon

• Starting on the next line, an indented block of code (called the

elif

clause)

Let’s add an

elif

to the name checker to see this statement in action.

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, kiddo.')

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Flow Control

This time, you check the person’s age, and the program will tell them

something different if they’re younger than 12. You can see the ﬂowchart

for this in Figure 2-5.

print('Hi, Alice.')

Start

End

name == 'Alice'

print('You are not Alice, kiddo.')

age < 12

True

False

True

False

Figure 2-5: The ﬂowchart for an elif statement

The

elif

clause executes if

age < 12

True

and

name == 'Alice'

False

However, if both of the conditions are

False

, then both of the clauses are

skipped. It is not guaranteed that at least one of the clauses will be exe-

cuted. When there is a chain of

elif

statements, only one or none of the

clauses will be executed. Once one of the statements’ conditions is found

to be

True

, the rest of the

elif

clauses are automatically skipped. For example,

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

vampire.py:

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, kiddo.')

elif age > 2000:

print('Unlike you, Alice is not an undead, immortal vampire.')

elif age > 100:

print('You are not Alice, grannie.')

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Chapter 2

Here I’ve added two more

elif

statements to make the name checker

greet a person with different answers based on

age

. Figure 2-6 shows the

ﬂowchart for this.

print('Hi, Alice.')

End

name == 'Alice'

print('You are not Alice, kiddo.')

age < 12

True

False

True

False

age > 100

True

False

print('You are not Alice, grannie.')

age > 2000

True

False

Start

print('Unlike you, Alice is not

an undead, immortal vampire.')

Figure 2-6: The ﬂowchart for multiple elif statements in the vampire‮py program

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Flow Control

The order of the

elif

statements does matter, however. Let’s rearrange

them to introduce a bug. Remember that the rest of the

elif

clauses are

automatically skipped once a

True

condition has been found, so if you swap

around some of the clauses in vampire.py, you run into a problem. Change

the code to look like the following, and save it as vampire2.py:

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, kiddo.')

u elif age > 100:

print('You are not Alice, grannie.')

elif age > 2000:

print('Unlike you, Alice is not an undead, immortal vampire.')

Say the

age

variable contains the value

3000

before this code is executed.

You might expect the code to print the string

'Unlike you, Alice is not

an undead, immortal vampire.'

. However, because the

age > 100

condition is

True

(after all, 3000 is greater than 100) u, the string

'You are not Alice,

grannie.'

is printed, and the rest of the

elif

statements are automatically

skipped. Remember, at most only one of the clauses will be executed, and

for

elif

statements, the order matters!

Figure 2-7 shows the ﬂowchart for the previous code. Notice how the

diamonds for

age > 100

and

age > 2000

are swapped.

Optionally, you can have an

else

statement after the last

elif

statement.

In that case, it is guaranteed that at least one (and only one) of the clauses

will be executed. If the conditions in every

and

elif

statement are

False

then the

else

clause is executed. For example, let’s re-create the Alice pro-

gram to use

elif

, and

else

clauses.

if name == 'Alice':

print('Hi, Alice.')

elif age < 12:

print('You are not Alice, kiddo.')

else:

print('You are neither Alice nor a little kid.')

Figure 2-8 shows the ﬂowchart for this new code, which we’ll save as

littleKid.py.

In plain English, this type of ﬂow control structure would be, “If the

ﬁrst condition is true, do this. Else, if the second condition is true, do that.

Otherwise, do something else.” When you use all three of these statements

together, remember these rules about how to order them to avoid bugs like

the one in Figure 2-7. First, there is always exactly one

statement. Any

elif

statements you need should follow the

statement. Second, if you want to

be sure that at least one clause is executed, close the structure with an

else

statement.

Chapter 2

print('Hi, Alice.')

Start

End

print('You are not Alice, kiddo.')

True

False

True

False

print('Unlike you, Alice is not

an undead, immortal vampire.')

True

False

print('You are not Alice, grannie.')

False

name == 'Alice'

age < 12

age > 2000

age > 100

True

Figure 2-7: The ﬂowchart for the vampire2‮py program. The crossed-out path

will logically never happen, because if age were greater than 2000, it would

have already been greater than 100.

Flow Control

print('Hi, Alice.')

Start

End

name == 'Alice'

print('You are not Alice, kiddo.')

age < 12

True

False

True

False

print('You are neither Alice

nor a little kid.')

Figure 2-8: Flowchart for the previous littleKid‮py program

while Loop Statements

You can make a block of code execute over and over again with a

while

state-

ment. The code in a

while

clause will be executed as long as the

while

state-

ment’s condition is

True

. In code, a

while

statement always consists of the

following:

• The

while

keyword

• A condition (that is, an expression that evaluates to

True

False

)

• A colon

• Starting on the next line, an indented block of code (called the

while

clause)

Chapter 2

You can see that a

while

statement looks similar to an

statement. The

difference is in how they behave. At the end of an

clause, the program

execution continues after the

statement. But at the end of a

while

clause,

the program execution jumps back to the start of the

while

statement. The

while

clause is often called the while loop or just the loop.

Let’s look at an

statement and a

while

loop that use the same condi-

tion and take the same actions based on that condition. Here is the code

with an

statement:

spam = 0

if spam < 5:

print('Hello, world.')

spam = spam + 1

Here is the code with a

while

statement:

spam = 0

while spam < 5:

print('Hello, world.')

spam = spam + 1

These statements are similar—both

and

while

check the value of

spam

and if it’s less than ﬁve, they print a message. But when you run these two

code snippets, something very different happens for each one. For the

statement, the output is simply

"Hello, world."

. But for the

while

statement,

it’s

"Hello, world."

repeated ﬁve times! Take a look at the ﬂowcharts for

these two pieces of code, Figures 2-9 and 2-10, to see why this happens.

print('Hello, world.')

Start

End

spam < 5

spam = spam + 1

True

False

Figure 2-9: The ﬂowchart for the if statement code

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