Techniques to Exit Nested Loops in Python

Python provides two loop structures, while and for . When you need to terminate loops early, the break statement works, but breaking out of multiple nested loops requires additional techniques.

We illustrate several methods using a simple prime-number game where the user repeatedly inputs numbers; the game ends when a non‑prime is entered.

Using identical break conditions in each loop

This approach places a break in both the inner and outer loops, checking for a divisor and exiting when found.

<code># 此程序仅为演示相同条件跳出循环，输入2的时候会报错
while True:
    number = int(input('Please input an integer: '))
    for i in range(2, number):
        if number % i == 0:
            break
    if number % i == 0:
        break
print('Game Over')</code>

Using a flag variable

A flag is set to indicate whether the inner loop found a divisor; after the inner loop, the outer loop checks the flag and breaks accordingly.

<code>while True:
    number = int(input('Please input an integer: '))
    flag = True # 默认number是质数
    for i in range(2, number):
        if number % i == 0:
            flag = False
            break
    if not flag:
        break
print('Game Over')</code>

Raising an exception

When break is insufficient, you can raise and catch an exception (e.g., StopIteration ) to exit multiple loops.

<code>try:
    while True:
        number = int(input('Please input an integer: '))
        for i in range(2, number):
            if number % i == 0:
                raise StopIteration
except StopIteration:
    print('Game Over')</code>

Combining with else

Python's loop‑ else construct can be used so that the outer loop’s else runs only when the inner loop does not break, allowing controlled exit.

<code>while True:
    number = int(input('Please input an integer: '))
    for i in range(2, number):
        if number % i == 0:
            break
    else:
        continue
    break
print('Game Over')</code>

Using return inside a function

Encapsulating the nested loops in a function lets a return statement terminate the loops and exit the function.

<code>def prime_game():
    while True:
        number = int(input('Please input an integer: '))
        for i in range(2, number):
            if number % i == 0:
                return

prime_game()
print('Game Over')</code>

These techniques provide flexible ways to break out of nested loops in Python.