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Python while Loop (Infinite Loop, break, continue)

Modified: | Tags: Python

This article explains a while loop in Python.

Unlike a for loop, which sequentially processes iterable elements such as a list, a while loop repeats as long as its condition evaluates to True.

Contents

See the following article for information on a for loop. A for loop is better suited when you need to process elements from iterables, such as a list, or when you want to execute a loop a specific number of times.

An infinite loop can be implemented using a for loop and the functions of the itertools module instead of using a while loop. Infinite loops with counters and similar use cases can often be written more easily using these functions.

Basic syntax of while loops in Python

A while loop in Python is written as follows:

while condition:
    ... <do something>

i = 0

while i < 3:
    print(i)
    i += 1
# 0
# 1
# 2
source: while_usage.py

You can specify multiple conditions for the condition part with and or or.

Break a while loop: break

Use break to break a while loop.

i = 0

while i < 3:
    print(i)
    if i == 1:
        print('!!BREAK!!')
        break
    i += 1
# 0
# 1
# !!BREAK!!
source: while_usage.py

Continue to the next iteration: continue

You can skip the current iteration and move to the next one using the continue statement. break terminates the entire while loop, whereas continue only skips the remaining code in the current iteration.

while i < 3:
    if i == 1:
        print('!!CONTINUE!!')
        i += 1
        continue
    print(i)
    i += 1
# 0
# !!CONTINUE!!
# 2
source: while_usage.py

Note that if you write code that updates a variable used in the condition, such as i += 1, after the continue statement, the variable will not be updated. As a result, the loop may not terminate as expected.

Details on forced termination are described at the end.

Execute code after normal termination: else

Use else to execute something after the while loop has been completed successfully.

i = 0

while i < 3:
    print(i)
    i += 1
else:
    print('!!FINISH!!')
# 0
# 1
# 2
# !!FINISH!!
source: while_usage.py

If the loop is terminated by break, the code block in the else clause will not be executed.

i = 0

while i < 3:
    print(i)
    if i == 1:
        print('!!BREAK!!')
        break
    i += 1
else:
    print('!!FINISH!!')
# 0
# 1
# !!BREAK!!
source: while_usage.py

When the loop completes normally, even if continue has been used within the loop, the code block in the else clause will still be executed.

i = 0

while i < 3:
    if i == 1:
        print('!!SKIP!!')
        i += 1
        continue
    print(i)
    i += 1
else:
    print('!!FINISH!!')
# 0
# !!SKIP!!
# 2
# !!FINISH!!
source: while_usage.py

Infinite loop with while statement: while True:

If the condition in the while statement is always True, the loop will never end, and execution will repeat infinitely.

In the following example, the Unix time is acquired using time.time(). The elapsed time is then measured and used to set the condition for when to break out of the loop.

import time

start = time.time()

while True:
    time.sleep(1)
    print('processing...')
    if time.time() - start > 5:
        print('!!BREAK!!')
        break
# processing...
# processing...
# processing...
# processing...
# processing...
# !!BREAK!!
source: while_usage.py

For example, since non-zero numbers and non-empty strings are considered True, a statement like while 1: will create an infinite loop.

start = time.time()

while 1:
    time.sleep(1)
    print('processing...')
    if time.time() - start > 5:
        print('!!BREAK!!')
        break
# processing...
# processing...
# processing...
# processing...
# processing...
# !!BREAK!!
source: while_usage.py

In the sample code above, an infinite loop is used for explanation; however, the same operation can be performed without an infinite loop, as shown below.

start = time.time()

while time.time() - start <= 5:
    time.sleep(1)
    print('processing...')
else:
    print('!!FINISH!!')
# processing...
# processing...
# processing...
# processing...
# processing...
# !!FINISH!!
source: while_usage.py

Stop the infinite loop using keyboard interrupt (ctrl + c)

To stop the infinite loop using keyboard interrupt instead of setting break, you can catch the KeyboardInterrupt.

try:
    while True:
        time.sleep(1)
        print('processing...')
except KeyboardInterrupt:
    print('!!FINISH!!')
source: while_keyboardinterrupt.py

If you press ctrl + c in the running terminal (Mac) or command prompt (Windows cmd.exe), the while loop will be terminated, and the except clause will be executed.

See the following article for exception handling.

Forced termination

If you make a mistake in setting the condition, the process may fall into an infinite loop and never end.

It can be forcibly terminated in the following ways.

  • Press ctrl + c in the running terminal
  • End the process in Activity Monitor or Task Manager
    • Find the process name Python and kill it

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