Python Tutorial — Learn Python Step by Step

Python for Loop

The for loop iterates over sequences (lists, strings, ranges, dicts) — it is Python's primary iteration tool.

Basic for Loop with range()

# range(stop) — 0 to stop-1
for i in range(5):
    print(i, end=" ")  # 0 1 2 3 4

# range(start, stop, step)
for i in range(2, 10, 2):
    print(i, end=" ")  # 2 4 6 8

# Counting backwards
for i in range(10, 0, -1):
    print(i, end=" ")  # 10 9 8 ... 1

Iterating Collections

# List
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# String (character by character)
for char in "Python":
    print(char, end="-")  # P-y-t-h-o-n-

# Dictionary
user = {"name": "Alice", "age": 25, "city": "NYC"}
for key, value in user.items():
    print(f"{key}: {value}")

# Enumerate — get index AND value
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple, 1: banana, 2: cherry

# Zip — iterate multiple sequences together
names = ["Alice", "Bob"]
scores = [95, 87]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

List Comprehension (One-line for)

# [expression for item in iterable if condition]
squares = [x**2 for x in range(10)]
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

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

# Dict comprehension
word_lengths = {w: len(w) for w in ["hello", "world", "python"]}
# {"hello": 5, "world": 5, "python": 6}

• Use enumerate() when you need the index during iteration.
• Use zip() to iterate multiple sequences in parallel.
• List comprehensions are Pythonic and faster than equivalent for loops.