for

How it works

Iterate directly over values when possible. Use enumerate() when you also need a counter, and dictionary.items() when you need each key and value. A dictionary preserves insertion order in Python 3.7 and later; a set does not promise a useful display order.

The body runs once per item and zero times for an empty iterable. When an iterator is exhausted, the loop finishes. A generator is consumed as it is iterated, so a second loop over the same exhausted generator produces no items.

In nested loops, the inner loop runs for each outer item. Three outer iterations with three inner iterations produce nine pairs. Consider how work grows before nesting loops over large collections.

Common mistakes and boundaries

Changing a collection’s structure while iterating it can skip values or raise an error. Build a new collection or iterate an appropriate copy when adding or removing items.

The loop target remains bound after a nonempty loop; if the loop never ran, it may never have been assigned. Do not depend on a final loop value without handling the empty case.

Try it yourself

Use enumerate to print a numbered shopping list. Test an empty list, then a one-item list. Next filter out blank entries into a new list rather than removing items during iteration.

• range()
• break and continue
• Generators (yield)
• List Comprehensions

Syntax

for variable in iterable:
    # code to execute for each item

Examples

fruits = ["apple", "banana", "orange", "grape"]

for fruit in fruits:
    print(f"I like {fruit}")

numbers = [1, 2, 3, 4, 5]
total = 0

for num in numbers:
    total += num

print(f"Sum: {total}")  # 15

# Count from 0 to 4
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

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

# Count by steps
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# Countdown
for i in range(5, 0, -1):
    print(f"{i}...")  # 5... 4... 3... 2... 1...
print("Liftoff!")

message = "Python"

for char in message:
    print(char)  # P, y, t, h, o, n

# Count vowels
text = "Hello World"
vowels = "aeiouAEIOU"
vowel_count = 0

for char in text:
    if char in vowels:
        vowel_count += 1

print(f"Vowels found: {vowel_count}")

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

for index, fruit in enumerate(fruits):
    print(f"{index + 1}. {fruit}")
# Output:
# 1. apple
# 2. banana
# 3. orange

# Start counting from different number
for index, fruit in enumerate(fruits, start=10):
    print(f"Item {index}: {fruit}")

student_scores = {
    "Alice": 85,
    "Bob": 92,
    "Charlie": 78
}

# Iterate over keys
for name in student_scores:
    print(f"{name}: {student_scores[name]}")

# Iterate over values
for score in student_scores.values():
    print(f"Score: {score}")

# Iterate over key-value pairs
for name, score in student_scores.items():
    print(f"{name} scored {score}")
    if score >= 90:
        print("  Excellent!")

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        product = i * j
        print(f"{i} × {j} = {product}")
    print()  # Empty line between tables

# Pattern printing
for row in range(1, 5):
    for col in range(row):
        print("*", end=" ")
    print()  # New line after each row

# Break - exit loop early
for num in range(1, 11):
    if num == 5:
        print("Found 5! Stopping...")
        break
    print(num)

print()

# Continue - skip to next iteration
for num in range(1, 6):
    if num == 3:
        continue  # Skip 3
    print(num)  # Prints 1, 2, 4, 5

print()

# Filter and stop using ordinary task labels
for task in ["", "draft", "review", "publish"]:
    if not task:
        continue
    if task == "review":
        print("Found review")
        break
    print(f"Checked: {task}")

Best practices

• Use descriptive variable names in for loops (for student in students: instead of for s in students:)
• Use enumerate() when you need both index and value
• Prefer list comprehensions for simple transformations ([x*2 for x in numbers])
• Avoid modifying the list you're iterating over - create a new list instead
• Use range(len(list)) only when absolutely necessary - prefer direct iteration
• Break complex nested loops into separate functions for better readability

Runtime & versions

Python interpreter. Feature availability depends on the language and runtime version used by your project; consult the official reference below.

Related Python documentation

Specifications & further reading

• for statements
• Looping techniques