Conditionals

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Conditional statements let you execute code conditionally based on some condition; they are similar in nature to the while loop but only run at most once.

In Python, the keywords to control these conditionals are if , elif (read “else if”) and else . For those that know Java, these keywords exactly match the semantics of Java’s if , else if and else keywords.

A conditional block is an if block optionally followed by any number of elif blocks optionally followed by at most one else block.

x = 14
if x < 10:
    print('A')
elif x >= 13:
    print('B')
elif x > 20:
    print('Not possible')
else:
    print('C')

This code prints B because the first if test fails ( x < 10 is False ) and then the second test succeeds ( x >= 13 is True ) so we enter the second body.

Two questions you should consider (you should think about these questions before expanding the output to see the answer!):

• What values of x enter the else statement?

Output

The only values of x that can cause the code to enter the else block are 10 , 11 , and 12 . Any number less than 10 will enter the first if while any number greater than 12 will enter the first elif.

• Why is it not possible to enter the third block (the second elif block)?

Output

Using the logic from the previous question, no value of x can satisfy this condition if it doesn’t satisfy the previous two. To enter the second elif block, x would need to be some value >= 10 , < 13 , and >= 20 . No such number exists!