Omitting the else Clause: Unary Selection

Section 7.5 Omitting the else Clause: Unary Selection

Another form of the if statement is one in which the else clause is omitted entirely. This creates what is sometimes called unary selection . In this case, when the condition evaluates to True , the statements are executed. Otherwise the flow of execution continues to the statement after the body of the if .

Listing 7.5.1.

What would be printed if the value of x is negative? Try it.

Check your understanding

Checkpoint 7.5.2.

What does the following code print?

x = -10
if x < 0:
    print("The negative number ",  x, " is not valid here.")
print("This is always printed")

a.
This is always printed

b.
The negative number -10 is not valid here
This is always printed

c.
The negative number -10 is not valid here

Output a
Because -10 is less than 0, Python will execute the body of the if-statement here.
Output b
Python executes the body of the if-block as well as the statement that follows the if-block.
Output c
Python will also execute the statement that follows the if-block (because it is not enclosed in an else-block, but rather just a normal statement).
It will cause an error because every if must have an else clause.
It is valid to have an if-block without a corresponding else-block (though you cannot have an else-block without a corresponding if-block).

Checkpoint 7.5.3.

Will the following code cause an error?

x = -10
if x < 0:
    print("The negative number ",  x, " is not valid here.")
else:
    print(x, " is a positive number")
else:
    print("This is always printed")

No
Every else-block must have exactly one corresponding if-block. If you want to chain if-else statements together, you must use the else if construct, described in the chained conditionals section.
Yes
This will cause an error because the second else-block is not attached to a corresponding if-block.