Python Loops Tutorial - for, while, range, enumerate Examples

What are Loops?

Loops let you execute a block of code repeatedly. Python has two loop types: for (iterate over a sequence) and while (repeat while a condition is true).

for Loop

Iterates over any iterable - list, tuple, string, range, dict, etc.

for Loop

# Iterate over a list
fruits = ["apple", "banana", "mango"]
for fruit in fruits:
    print(fruit)

# Iterate over a string
for char in "Python":
    print(char, end=" ")  # P y t h o n

# Iterate over a range
for i in range(5):        # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):     # 1, 2, 3, 4, 5
    print(i)

for i in range(0, 10, 2): # 0, 2, 4, 6, 8 (step=2)
    print(i)

for i in range(10, 0, -1): # 10, 9, 8 ... 1 (countdown)
    print(i)

Useful for Loop Patterns

enumerate, zip, items

fruits = ["apple", "banana", "mango"]

# enumerate - get index and value
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple
# 1: banana
# 2: mango

# enumerate with custom start
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

# zip - iterate two lists together
names = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 92]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

# Iterate dict items
person = {"name": "Alice", "age": 25}
for key, value in person.items():
    print(f"{key} = {value}")

# Nested loops
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i}x{j}={i*j}", end="  ")
    print()

while Loop

Repeats as long as a condition is True. Use when you don't know the number of iterations in advance.

while Loop

count = 0
while count < 5:
    print(count)
    count += 1   # IMPORTANT: always update the condition variable

# User input loop
while True:
    answer = input("Type 'quit' to exit: ")
    if answer == "quit":
        break
    print(f"You typed: {answer}")

# Countdown
n = 10
while n > 0:
    print(n, end=" ")
    n -= 1
print("Go!")

Infinite Loop: Always make sure the while condition eventually becomes False, or use break to exit.

break, continue, pass

Loop Control

# break - exit the loop immediately
for i in range(10):
    if i == 5:
        break
    print(i)   # prints 0 1 2 3 4

# continue - skip current iteration, go to next
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)   # prints 1 3 5 7 9 (odd numbers only)

# pass - do nothing (placeholder)
for i in range(5):
    if i == 3:
        pass   # TODO: handle this case later
    print(i)   # prints all 0 1 2 3 4

# for-else / while-else
# else block runs only if loop completed without break
for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            break
    else:
        print(f"{n} is prime")  # 2 3 5 7

List Comprehensions

A concise way to create lists using a single line. Much more Pythonic than a for loop with append().

Comprehensions

# List comprehension: [expression for item in iterable if condition]
squares = [x**2 for x in range(1, 6)]
print(squares)   # [1, 4, 9, 16, 25]

evens = [x for x in range(20) if x % 2 == 0]
print(evens)     # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

words = ["hello", "world", "python"]
upper = [w.upper() for w in words]
print(upper)     # ['HELLO', 'WORLD', 'PYTHON']

# Nested comprehension (flatten a 2D list)
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [num for row in matrix for num in row]
print(flat)      # [1, 2, 3, 4, 5, 6, 7, 8, 9]

# Dict comprehension
squares_dict = {x: x**2 for x in range(1, 6)}
print(squares_dict)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# Set comprehension
unique_lengths = {len(w) for w in words}
print(unique_lengths)  # {5, 6}