Python for Loop

In Python, we use a for loop to iterate over sequences such as lists, strings, dictionaries, etc. For example,

languages = ['Swift', 'Python', 'Go']

# access elements of the list one by one
for lang in languages:
    print(lang)

Output

Swift
Python
Go

In the above example, we have created a list named languages. Since the list has three elements, the loop iterates 3 times.

The value of lang is

• Swift in the first iteration.
• Python in the second iteration.
• Go in the third iteration.

for loop Syntax

for val in sequence:
    # run this code

The for loop iterates over the elements of sequence in order, and in each iteration, the body of the loop is executed.

The loop ends after the body of the loop is executed for the last item.

Indentation in Loop

In Python, we use indentation (spaces at the beginning of a line) to define a block of code, such as the body of a loop. For example,

languages = ['Swift', 'Python', 'Go']

# start of the loop
for lang in languages:
    print(lang)
    print('-----')
# end of the for loop

print('Last statement')

Output

Swift
-----
Python
-----
Go
-----
Last statement

Here, print('Last statement') is outside the body of the loop. Therefore, this statement is executed only once at the end.

Example: Loop Through a String

If we iterate through a string, we get individual characters of the string one by one.

language = 'Python'

# iterate over each character in language
for x in language:
    print(x)

Output

P
y
t
h
o
n

Here, we have printed each character of the string language using a for loop.

for Loop with Python range()

In Python, the range() function returns a sequence of numbers. For example,

# generate numbers from 0 to 3
values = range(0, 4)

Here, range(0, 4) returns a sequence of 0, 1, 2 ,and 3.

Since the range() function returns a sequence of numbers, we can iterate over it using a for loop. For example,

# iterate from i = 0 to i = 3
for i in range(0, 4):
    print(i)

Output

0
1
2
3

Here, we used the for loop to iterate over a range from 0 to 3.

This is how the above program works.

break and continue Statement

The break and continue statements are used to alter the flow of loops.

The break Statement

The break statement terminates the for loop immediately before it loops through all the items. For example,

languages = ['Swift', 'Python', 'Go', 'C++']

for lang in languages:
    if lang == 'Go':
        break
    print(lang)

Output

Swift
Python

Here, when lang is equal to 'Go', the break statement inside the if condition executes which terminates the loop immediately. This is why Go and C++ are not printed.

The continue Statement

The continue statement skips the current iteration of the loop and continues with the next iteration. For example,

languages = ['Swift', 'Python', 'Go', 'C++']

for lang in languages:
    if lang == 'Go':
        continue
    print(lang)

Output

Swift
Python
C++

Here, when lang is equal to 'Go', the continue statement executes, which skips the remaining code inside the loop for that iteration.

However, the loop continues to the next iteration. This is why C++ is displayed in the output.

Visit Python break and continue article to learn more.

Nested for loops

A loop can also contain another loop inside it. These loops are called nested loops.

In a nested loop, the inner loop is executed once for each iteration of the outer loop.

# outer loop 
attributes = ['Electric', 'Fast']
cars = ['Tesla', 'Porsche', 'Mercedes']

for attribute in attributes:
    for car in cars:
        print(attribute, car)
    
    # this statement is outside the inner loop
    print("-----")
  

Output

Electric Tesla
Electric Porsche
Electric Mercedes
-----
Fast Tesla
Fast Porsche
Fast Mercedes
-----

Using for loop without accessing sequence items

If we don't intend to use items of sequence inside the body of a loop, it is clearer to use the _ (underscore) as the loop variable. For example,

# iterate from i = 0 to 3
for _ in range(0, 4:
    print('Hi')

Output

0
1
2
3

Here, the loop runs four times. In each iteration, we have displayed Hi. Since we are not using the items of the sequence(0, 1, 2 and 4) in the loop body, it is better to use _ as the loop variable.

Also read: Python while loop