For Loop

Let's start with an example.

Suppose we have a list of animals and we want to great each one of them. To begin with, we'll simply print each animal:

animals = ['cat', 'dog', 'monkey']

for animal in animals: # assign next value to animal
    print(animal)      # print animal

The first line of this program should be familiar by now; it creates a list of three animals.

What happens next is the interesting part. We use a for loop to iterate over the list of animals. Each time the loop executes, the variable animal will be assigned the next value in the list. The first time through the loop, animal will be assigned the value 'cat', the second time through the loop, animal will be assigned the value 'dog', and the third time through the loop, animal will be assigned the value 'monkey'. The loop will terminate when there are no more values in the list.

At every step the loop performs the same action: it prints the value of animal. That's easy to see from the output of the program:

cat
dog
monkey
Hello, Cat!
Hello, Dog!
Hello, Monkey!
Hello, Monkey!

Note that the action performed by the loop is indented. This is a requirement in Python. The indentation tells Python which statements are part of the loop. In this case we have a single statement, the print statement, but we could have had several statements, all indented to the same level.

An example of a program with few statements is given below:

for animal in animals:
    greeting = f"Hello, {animal.title()}!"
    print(greeting)

with the output:

Hello, Cat!
Hello, Dog!
Hello, Monkey!

If we don't indent the second statements in the loop, we won't get any error. This still will be a valid Python program, but it will do something different:

for animal in animals:
    greeting = f"Hello, {animal.title()}!"
print(greeting)

The result of executing this program will be a single line, with the last value of greeting:

Hello, Monkey!