for else in Python

A for loop can have an else block. The else runs when the loop has gone through every item. If the loop ends with break, the else is skipped. This makes it a good fit for searching: break when you find the item, and use else for "not found".

The program

def search(nums, target):
    for i, n in enumerate(nums):
        if n == target:
            print(f"Found {target} at index {i}")
            break
    else:
        print(f"{target} is not in the list")

nums = [1, 3, 5, 7, 9]
search(nums, 7)
search(nums, 6)

Found 7 at index 3
6 is not in the list

How it works

• When the target is found, break ends the loop and the else block does not run.
• When the loop checks every item without finding it, there was no break, so the else block runs.
• Read the else as "no break". It belongs to the for, not to the if inside it, so it is indented at the same level as for.

The same search with a flag

Without for-else, you need an extra variable to remember whether the item was found:

nums = [1, 3, 5, 7, 9]
target = 6

found = False
for n in nums:
    if n == target:
        found = True
        break

if not found:
    print(f"{target} is not in the list")

6 is not in the list

A prime number check

for-else also fits checks like "does any number divide this one?":

for n in [7, 9, 13]:
    for d in range(2, n):
        if n % d == 0:
            print(f"{n} is not prime ({d} x {n // d})")
            break
    else:
        print(f"{n} is prime")

7 is prime
9 is not prime (3 x 3)
13 is prime

To only check whether a value is in a list, if target in nums: is shorter. for-else is for when the loop does other work too, such as finding the index or the divisor.

Related programs: while loop with else.