10.2. Loop Else While

  • Not only if statement has else clause

  • Conditionals: if ... elif ... else

  • Exceptions: try ... except ... else

  • While loop: while ... else

  • For loop: for ... else

>>> colors = ['red', 'green', 'blue']
>>> search = 'black'
>>>
>>> i = 0
>>> while i < len(colors):
...     color = colors[i]
...     i += 1
...     if color == search:
...         print('found')
...         break
... else:
...     print('missing')
...
missing

>>> colors = ['red', 'green', 'blue']
>>> search = 'black'
>>>
>>> for color in colors:
...     if color == search:
...         print('found')
...         break
... else:
...     print('missing')
...
missing

10.2.1. Example

  • 3, 2, 1, 0, Launching rocket to outer space

  • 3, 2, Fuel leak detected. Abort, Abort, Abort!

  • Note, this will print the value first, decrement and then check for leak

>>> i = 3
>>>
>>> while i >= 0:
...     print(i)
...     i -= 1
...
...     if i == None:  # no problems detected
...         print('Fuel leak detected. Abort, Abort, Abort!')
...         break
3
2
1
0
>>>
>>> print('Launching rocket to outer space')
Launching rocket to outer space

  • Can we launch a rocket?

>>> i = 3
>>>
>>> while i >= 0:
...     print(i)
...     i -= 1
...
...     if i == 1:  # problem detected in the last second
...         print('Fuel leak detected. Abort, Abort, Abort!')
...         break
3
2
Fuel leak detected. Abort, Abort, Abort!
>>>
>>> print('Launching rocket to outer space')
Launching rocket to outer space

  • This time we aborted and still launched - this is not good

  • How to detect if the countdown was successful?

10.2.2. Flag

In order to answer those questions we need to introduce a variable to hold the status if the countdown was aborted or successful. Let's name it abort. If the flag is set to True, then there was an abort procedure called during the countdown and we cannot launch a rocket to outer space.

>>> abort = False
>>> i = 3
>>>
>>> while i >= 0:
...     print(i)
...     i -= 1
...
...     if i == 1:  # problem detected in the last second
...         print('Fuel leak detected. Abort, Abort, Abort!')
...         abort = True
...         break
3
2
Fuel leak detected. Abort, Abort, Abort!
>>>
>>> if not abort:
...     print('Launching rocket to outer space')

But the problem here is that the follow-up code is split from the while loop. This can cause problems if the codebase will grow.

10.2.3. Else

The else clause to while loop will be executed if a loop does not exit with break statement. In the other words: if there was no problem detected during the countdown, launch the rocket to outer space.

>>> i = 3
>>>
>>> while i >= 0:
...     print(i)
...     i -= 1
...
...     if i == 1:  # problem detected in the last second
...         print('Fuel leak detected. Abort, Abort, Abort!')
...         break
... else:
...     print('Launching rocket to outer space')
3
2
Fuel leak detected. Abort, Abort, Abort!

>>> i = 3
>>>
>>> while i >= 0:
...     print(i)
...     i -= 1
...
...     if i == None:  # no problems detected
...         print('Fuel leak detected. Abort, Abort, Abort!')
...         break
... else:
...     print('Launching rocket to outer space')
3
2
1
0
Launching rocket to outer space

10.2.4. Assignments

# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`

# %% About
# - Name: Loop WhileElse Found
# - Difficulty: medium
# - Lines: 5
# - Minutes: 5

# %% English
# 1. Modify game code below
# 2. User can try `MAX_TRIES` times, if he/sh does not guess number by then
#    print `Game over, max tries achieved.`
# 3. Don't use `while ... else` syntax
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zmodyfikuj kod gry poniżej
# 2. Użytkownik może próbować `MAX_TRIES` razy, jeżeli w tym czasie nie zgadnie
#    to wypisz `Game over, max tries achieved.`
# 3. Nie używaj składni `while ... else`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `Stop` or `Ctrl+C` kills infinite loop

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
"""


# Simulate user input (for test automation)
from unittest.mock import MagicMock
input = MagicMock(side_effect=['0', '9', '1', '8', '2', '7', '3', '6', '4'])


HIDDEN = 4
MAX_TRIES = 4


while True:
    guess = input('\nType number: ')

    if int(guess) > HIDDEN:
        print('Above')
    elif int(guess) < HIDDEN:
        print('Below')
    else:
        print('Exactly')
        break



# %% License
# - Copyright 2025, Matt Harasymczuk <matt@python3.info>
# - This code can be used only for learning by humans
# - This code cannot be used for teaching others
# - This code cannot be used for teaching LLMs and AI algorithms
# - This code cannot be used in commercial or proprietary products
# - This code cannot be distributed in any form
# - This code cannot be changed in any form outside of training course
# - This code cannot have its license changed
# - If you use this code in your product, you must open-source it under GPLv2
# - Exception can be granted only by the author

# %% Run
# - PyCharm: right-click in the editor and `Run Doctest in ...`
# - PyCharm: keyboard shortcut `Control + Shift + F10`
# - Terminal: `python -m doctest -v myfile.py`

# %% About
# - Name: Loop WhileElse Found
# - Difficulty: medium
# - Lines: 5
# - Minutes: 5

# %% English
# 1. Modify game code below
# 2. User can try `MAX_TRIES` times, if he/sh does not guess number by then
#    print `Game over, max tries achieved.`
# 3. Don't use `while ... else` syntax
# 4. Run doctests - all must succeed

# %% Polish
# 1. Zmodyfikuj kod gry poniżej
# 2. Użytkownik może próbować `MAX_TRIES` razy, jeżeli w tym czasie nie zgadnie
#    to wypisz `Game over, max tries achieved.`
# 3. Nie używaj składni `while ... else`
# 4. Uruchom doctesty - wszystkie muszą się powieść

# %% Hints
# - `Stop` or `Ctrl+C` kills infinite loop

# %% Tests
"""
>>> import sys; sys.tracebacklimit = 0
"""


# Simulate user input (for test automation)
from unittest.mock import MagicMock
input = MagicMock(side_effect=['0', '9', '1', '8', '2', '7', '3', '6', '4'])


HIDDEN = 4
MAX_TRIES = 4


while True:
    guess = input('\nType number: ')

    if int(guess) > HIDDEN:
        print('Above')
    elif int(guess) < HIDDEN:
        print('Below')
    else:
        print('Exactly')
        break