The Python for-else Clause

Introduction

The Python for–else clause is an additional, optional syntax Python’s for loop supports. It enables you to execute a code block after the loop has finished normally, that is, after all the items in the loop’s iterable object had been iterated over. Abnormal, premature termination, due to reasons such as a break or return statement, skips that code.

Basic Python for–else Syntax

The Python for–else has the usual for loop syntax with the addition of the else keyword followed by a colon : and a code block:

for element in iterable:
    loop-block
else:
    else-block

• for element in iterable: The usual for loop syntax.
• loop-block: The code executed for each element in the for loop’s iterable.
• else-block: The code that executes after the loop finishes normally.

Using the else Clause With for Loops

The lesson about the break keyword introduced the code example below.

Example:

dogs = ["Scooby-Doo", "Snoopy", "Goofy", "Clifford", "Astro"]

num_of_treats = 3
for dog in dogs:
    if num_of_treats == 0:
        break 
    print(f"{dog} gets a treat. Woof!")
    num_of_treats -= 1

Output:

Scooby-Doo gets a treat.  Woof!
Snoopy gets a treat.  Woof!
Goofy gets a treat.  Woof!

As you may recall from that lesson, the example is about giving dogs some treats. The number of treats is limited, and not every dog may get one. The for loop iterates through the dogs (given by the dogs list), but if the treats run out, a break statement exits the loop and no more treats are given.

Now, additional requirements for the program came in, as follows:

1. Print an appropriate alert if the treats run out before all the dogs are given one.
2. Print an appropriate statement if all dogs were given a treat which includes include the number of treats remaining.

We’ll implement the first requirement by adding a print() statement right before the break in the same code block. We will implement the second requirement with an else clause.

Example:

dogs = ["Scooby-Doo", "Snoopy", "Goofy", "Clifford", "Astro"]

num_of_treats = 10
for dog in dogs:
    if num_of_treats == 0:
        print("The treats have run out! Please order some more.")
        break 
    print(f"{dog} gets a treat. Woof!")
    num_of_treats -= 1
else:
    print(f"Every dog got a treat. There are {num_of_treats} treats remaining. Good job!")

Output:

Scooby-Doo gets a treat. Woof!
Snoopy gets a treat. Woof!
Goofy gets a treat. Woof!
Clifford gets a treat. Woof!
Astro gets a treat. Woof!
Every dog got a treat. There are 5 treats remaining.  Good job!

As you can see in the example above, the first requirement to print an alert when the treats run out, is implemented by the print() statement on line 6. The second requirement to print a message when all dogs have been successfully given a treat, is implemented by the else clause and code block on lines 10 and 11. The code block of the else clause executes only if all dogs have been iterated through and the break statement on line 7 has not been executed.

Let’s see what happens when the break does execute. To do this we’ll change the initial number of treats to 2.

Example:

dogs = ["Scooby-Doo", "Snoopy", "Goofy", "Clifford", "Astro"]

num_of_treats = 2
for dog in dogs:
    if num_of_treats == 0:
        print("The treats have run out! Please order some more.")
        break 
    print(f"{dog} gets a treat. Woof!")
    num_of_treats -= 1
else:
    print(f"Every dog got a treat. There are {num_of_treats} treats remaining. Good job!")

Output:

Scooby-Doo gets a treat. Woof!
Snoopy gets a treat. Woof!
The treats have run out! Please order some more.

The only code change in the example above from the previous one is on line 3, where the number of treats is set to 2. But since there are five dogs, the treats run out, triggering the break on line 7. Because the loop does not terminate normally, the code block under the else is not executed.

The Alternative to the for Loop else Clause

The else clause exists because it can lead to less repetition and cleaner code. If we couldn’t use the else clause with the for loop, we would have to check the number of remaining treats again after the loop.

dogs = ["Scooby-Doo", "Snoopy", "Goofy", "Clifford", "Astro"]

num_of_treats = 2
for dog in dogs:
    if num_of_treats == 0:
        print("The treats have run out! Please order some more.")
        break 
    print(f"{dog} gets a treat. Woof!")
    num_of_treats -= 1

if num_of_treats != 0:
    print(f"Every dog got a treat. There are {num_of_treats} treats remaining. Good job!")

The example above has the same output as the previous one, but now it uses an if statement and a code block after the for loop on lines 11 and 12, instead of the else clause.

Summary & Reference for the Python for-else Clause

The Python for–else clause is an optional addition to the for loop. It executes a code block after the loop, if it finishes normally, which means after all the items in the loop’s iterable are iterated over.

Abnormal, premature loop termination, due to reasons such as a break or return statement, skips that code.

Here is the basic for–else syntax:

for element in iterable:
    loop-block
else:
    else-block