Python Loops

What are Python loops?

Loops repeat a block of code so you can process multiple values or perform a task until a condition changes. They reduce duplicated code and are useful for collections, validation, calculations, and repeated work.

Python for loop

A for loop takes one item at a time from an iterable and assigns it to the loop variable:

for number in range(1, 6):
    print(number)

The loop body is identified by indentation. The value of number changes on each iteration.

Iterating over sequences

A for loop can visit the items in a string, list, tuple, set, or dictionary:

for character in "Python":
    print(character)

languages = ["Python", "Java", "SQL"]
for language in languages:
    print(language)

student = {"name": "Asha", "experience": 8}
for key, value in student.items():
    print(key, value)

Iterating directly over items is usually clearer than managing indexes manually. Dictionary iteration uses keys by default; use .items() when both keys and values are needed.

Using range()

range() creates a sequence of numbers without storing every number as a list. Its stop value is exclusive:

print(list(range(5)))
print(list(range(2, 6)))
print(list(range(10, 4, -2)))

Getting an index with enumerate()

Use enumerate() when you need both the position and the item. It is more readable than looping over range(len(items)):

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

for index, fruit in enumerate(fruits, start=1):
    print(index, fruit)

Python while loop

A while loop repeats as long as its condition is true. Update the state used by the condition so the loop can eventually finish:

counter = 1

while counter <= 5:
    print(counter)
    counter += 1

Use a while loop for tasks such as retrying input, processing items until a queue is empty, or reading data until an end condition is reached.

else with loops

Python allows an else block after both for and while loops. The else block runs when the loop finishes normally. It does not run when the loop exits through break:

numbers = [2, 4, 6]

for number in numbers:
    if number % 2 != 0:
        print("Odd number found")
        break
else:
    print("All numbers are even")

This pattern is useful when searching for an item and needing a clear action when no item matches.

Nested loops

A nested loop is a loop inside another loop. The inner loop completes its iterations for every iteration of the outer loop:

for row in range(1, 4):
    for column in range(1, 4):
        print(row, column)
    print("row complete")

Nested loops are useful for grids and tables, but their work grows quickly. For large data sets, look for a more efficient algorithm or a library operation.

Loop control statements

for number in range(1, 6):
    if number == 2:
        continue
    if number == 5:
        break
    print(number)

for number in range(3):
    if number == 1:
        pass
    print("iteration", number)

Infinite loops

An infinite loop never reaches a false condition. It is usually caused by forgetting to update the loop state:

counter = 0

while counter < 3:
    print(counter)
    counter += 1

The example is safe because counter changes on every iteration. Avoid an accidental while True loop unless it has a deliberate exit condition, such as break.

Searching with a loop

users = ["Asha", "Ravi", "Mina"]
target = "Ravi"

for user in users:
    if user == target:
        print("User found")
        break
else:
    print("User not found")

Choosing the right loop

• Use for when iterating over items or a known numeric range.
• Use while when a condition controls how long the work continues.
• Use enumerate() instead of manual index counters when you need positions.
• Use break to stop a search once the result is known.
• Use continue to skip invalid or unwanted items.
• Keep loop bodies small and extract repeated logic into functions.
• Check the condition and update step carefully to avoid infinite loops.