Python For Loops

Learn how to use for loops in Python to iterate over sequences and collections.

Python For Loops

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Example

fruits = ["apple", "banana", "cherry"]
for x in fruits:
    print(x)

The for loop does not require an indexing variable to set beforehand.

Looping Through a String

Even strings are iterable objects, they contain a sequence of characters:

Example

for x in "banana":
    print(x)

The break Statement

With the break statement we can stop the loop before it has looped through all the items:

Example - Exit the loop when x is "banana":

fruits = ["apple", "banana", "cherry"]
for x in fruits:
    print(x)
    if x == "banana":
        break

Example - Exit the loop when x is "banana", but this time the break comes before the print:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
    if x == "banana":
        break
    print(x)

The continue Statement

With the continue statement we can stop the current iteration of the loop, and continue with the next:

Example - Do not print banana:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
    if x == "banana":
        continue
    print(x)

The range() Function

To loop through a set of code a specified number of times, we can use the range() function,

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

Example

for x in range(6):
    print(x)

Note that range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):

Example

for x in range(2, 6):
    print(x)

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

Example

for x in range(2, 30, 3):
    print(x)

Else in For Loop

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

Example

for x in range(6):
    print(x)
else:
    print("Finally finished!")

Note: The else block will NOT be executed if the loop is stopped by a break statement.

Example - Break the loop when x is 3, and see what happens with the else block:

for x in range(6):
    if x == 3: break
    print(x)
else:
    print("Finally finished!")

Nested Loops

A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer loop":

Example

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
    for y in fruits:
        print(x, y)

The pass Statement

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

Example

for x in [0, 1, 2]:
    pass

Practical Examples

Iterating Over Dictionary

student = {"name": "John", "age": 25, "grade": "A"}

# Iterate over keys
for key in student:
    print(key)

# Iterate over values
for value in student.values():
    print(value)

# Iterate over key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")

List Comprehension

# Traditional for loop
squares = []
for x in range(10):
    squares.append(x**2)

# List comprehension (more Pythonic)
squares = [x**2 for x in range(10)]
print(squares)

Enumerate Function

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

# Get both index and value
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Start enumeration from a different number
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}: {fruit}")

Zip Function

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
cities = ["New York", "London", "Tokyo"]

# Iterate over multiple lists simultaneously
for name, age, city in zip(names, ages, cities):
    print(f"{name} is {age} years old and lives in {city}")