Python has a few loop features that look unusual at first but make your code cleaner. Let's start with the else clause on loops.
The else clause
# Find a name in the list
names = ['Alice', 'Bob', 'Charlie']
for n in names:
if n == 'Bob':
print('Found Bob')
break
else:
# Runs ONLY if we did NOT hit break
print('No match found')
Classic search pattern
The else block runs when the loop finishes without hitting a break. If you do find something and call break, the else is skipped. This turns a common flag variable pattern into clean, readable code.
A while loop can have an else too, with the same rule: it runs when the condition becomes false, and is skipped when you leave with break.
The pass statement
# Python requires an indented block after a colon.
# If you have nothing to do there yet, use pass:
for n in [3, -1, 4]:
if n < 0:
pass # decide what to do with negatives later
else:
print(n)
A placeholder body
pass is a no-op. It satisfies Python's requirement for an indented block when you haven't written the real logic yet.
Nested loops
# A 2x3 grid
for row in range(2):
for col in range(3):
print(f'({row}, {col})')
# Output:
# (0, 0)
# (0, 1)
# (0, 2)
# (1, 0)
# (1, 1)
# (1, 2)
Rows and columns
Your turn
0 of 3 solved
Exercise 1
+30 XP
codes holds the codes that open the vault. Search it twice, each time with a for loop that has an else: first for first_try, then for second_try. When a loop finds the guess it prints f'{guess} opens the vault' (with the guess's value) and breaks; when it runs out of codes, its else prints f'{guess} does not'. With the starter's values the output is 4417 opens the vault and then 5555 does not.
codes = [1234, 4417, 9001]
first_try = 4417
second_try = 5555
# Search codes for first_try: a for loop with break, and an else
# Then search codes for second_try the same way
Ctrl + Enter to run
Run your code to check it against the tests.
Exercise 2
+30 XP
Add up readings, leaving the negative ones out. Loop over the readings with an if for a negative reading whose only statement is pass (Python needs at least one statement in every indented block, and pass is the one that does nothing), and an else that adds the reading to total. Print total after the loop.
readings = [4, -1, 7, -3, 2]
total = 0
# Loop over readings: pass for a negative reading, otherwise add it to total
Ctrl + Enter to run
Run your code to check it against the tests.
Exercise 3
+30 XP
shelves is a list of shelves, each a list of titles. Use a loop inside a loop to print every title as f'{shelf_number}: {title}', counting the shelves from 1, and add one to count for each title you print. A shelf is read only up to a title that is 'STOP': break out there without printing it. break only leaves the inner loop, so the next shelf is still read. The first line printed is 1: Dune.
shelves = [
['Dune', 'Emma', 'STOP', 'Ulysses'],
['Beloved', 'Hamlet'],
['STOP', 'Walden'],
['Persuasion'],
]
count = 0
shelf_number = 0
# For each shelf, add 1 to shelf_number, then print its titles up to 'STOP'