Section7.5Omitting 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.
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")
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.