5.9. Chained Conditionals

A chained conditional consists of a series of checks that are evaluated one after the other. If one check in the series evaluates to True, then all of the following checks are ignored.

5.9.1. elif Statements

If/else statements provide two alternative paths. A single condition determines which path to follow. We can build more complex conditionals using an elif clause. elif stands for else if, and it allows us to include more conditions and code blocks. This leads to more branches for our code to follow.

Example

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num = 10
other_num = 20

if num > other_num:
   print(num, "is greater than", other_num)
elif num < other_num:
   print(num, "is less than", other_num)
else:
   print(num, "is equal to", other_num)

Console Output

10 is less than 20

Summary:

  1. Line 4 begins the chained conditional. The boolean expression num > other_num returns False, since 10 is not greater than 20. This causes line 5 to be skipped.
  2. Line 6 contains the elif statement. The boolean expression num < other_num returns True, since 10 is less than 20. This triggers line 7.
  3. The code block for the else clause (line 9) is skipped, because one of the conditions above it was true.

Note

Just like with a simple if statement, the else clause is also optional for elif statements. In the example above, removing lines 8 & 9 will not break anything. However, the program will not print anything when num equals other_num, since the if and elif conditions both return False.

5.9.1.1. Multiple elif Statements

We can include more than one elif statement within a conditional. For example, the following code sample prints different messages depending on the first character in a string.

Example

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character = 'P'
lowercase = 'abcdefghijklmnopqrstuvwxyz'
uppercase = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
digits = '0123456789'

if character in lowercase:
   print(character, 'is a lowercase letter.')
elif character in uppercase:
   print(character, 'is an UPPERCASE letter.')
elif character in digits:
   print(character, 'is a number.')
else:
   print('&**%#!', character, 'is a punctuation mark or a space.')

We can easily add more elif statements to the conditional if we need to perform more checks. This gives us a huge amount of flexibility if we decide to modify the program later.

Here are some rules for building if/elif/else statements:

  1. Order is important. The if statement comes first, then the elif statements, and finally the else.
  2. The if statement and code block are required. The elif and else clauses are optional.
  3. Conditionals contain only ONE if statement and only ONE else (if used).
  4. Multiple elif statements are allowed after the if statement, but they must come before the else clause.

No matter how long we make a chained conditional, no more than one of the code blocks will run.

Example

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num = 10;
other_num = 20;

if num > other_num:
   print(num, "is greater than", other_num)
elif num < other_num:
   print(num, "is less than", other_num)
elif num % 5 == 0:
   print(num, "is divisible by 5")
elif num % 2 == 0:
   print(num, "is even")

Console Output

10 is less than 20

Even though both of the conditions num % 5 == 0 and num % 2 == 0 evaluate to True, neither line 9 nor 11 runs. Since line 6 is satisfied first, the rest of the conditional is skipped.

5.9.2. Nested vs. Chained Conditionals

On the previous page, we used a nested conditional to print different outputs based on the length of a word. We can accomplish the same result with a chained conditional.

Example

Here is the nested conditional again:

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word = input('Please enter a word: ')

if len(word) == 4:
   print("What did your mom tell you about using 4-letter words?")
else:
   if len(word) < 4:
      print("You can think of a longer word than that!")
   else:
      print("Excellent word!")

The following chained conditional produces the same result:

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word = input('Please enter a word: ')

if len(word) == 4:
   print("What did your mom tell you about using 4-letter words?")
elif len(word) < 4:
   print("You can think of a longer word than that!")
else:
   print("Excellent word!")

Often, you can use a nested conditional or a chained conditional to solve the same problem. Which one you choose depends on your personal preference, but you should always use the option that makes your code easier for others to read.

Example

Nesting one conditional inside of another performs a check within a check, and we can do this any number of times.

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if condition_1:
   # code here

   if condition_2:
      # code here

      if condition_3:
         # code here
      else:
         # code here

   else:
      # code here

else:
   # code here

Perhaps you see the problem with nesting more than once or twice. The code quickly gets very difficult to read and follow.

Even though we COULD code a check within a check within a check within a check within a check (etc.), we really SHOULDN’T. In most instances, we can make our code more readable by using chained conditionals and/or logical operators in place of nested conditionals.

5.9.3. Check Your Understanding

Question

What does the following code print?

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num = 8

if num % 2 == 0:
   print("Launch")
elif num > 5:
   print("Code")
else:
   print("LaunchCode")
  1. Launch
  2. Code
  3. Launch
    Code
  4. LaunchCode

Question

Examine this nested conditional:

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num = -10
if num < 0:
   print("The negative number", num, "is not valid here.")
else:
   if num > 0:
      print(num, "is a positive number")
   else:
      print(num, "is 0")

Which of the following code blocks gives the same result?

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    if num < 0:
       print("The negative number", num, "is not valid here.")
    else num > 0:
       print(num, "is a positive number")
    else:
       print(num, "is 0")
    
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    if num < 0:
       print("The negative number", num, "is not valid here.")
    elif num > 0:
       print(num, "is a positive number")
    else:
       print(num, "is 0")
    
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    if num < 0:
       print("The negative number", num, "is not valid here.")
    if num > 0:
       print(num, "is a positive number")
    else:
       print(num, "is 0")
    
  1. Code sample a
  2. Code sample b
  3. Code sample c