Python Loops: For, While, and Range Explained

Loops let you run the same code over and over without rewriting it each time. Whether you need to count through numbers, process items in a list, or keep going until a condition changes, Python gives you for loops and while loops to handle it.

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How Does a For Loop Work in Python?

Let's say you want to print the numbers 0 through 9. You could write ten print statements:

print(0)
print(1)
print(2)
# ... you get the idea
print(9)

A for loop saves you all that typing — especially if you wanted to do the same thing a thousand or a million times:

for i in range(0, 10):
    print(i)

The variable i takes on each value from 0 to 9, one at a time. In plain English:

1. Start with i equals 0
2. If i is greater than or equal to 10, exit the loop. Otherwise:
   • Print i to the console
   • Add 1 to i
   • Go back to step 2

The numbers in range(a, b) are inclusive of a and exclusive of b, so range(0, 10) includes 0 but not 10.

How Does range() Work in Python?

The range() function actually takes up to three parameters: range(start, stop, step).

The step parameter controls how much i changes each iteration. By default it's 1, but you can change it:

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

You can even go backwards with a negative step:

for i in range(3, 0, -1):
    print(i)
# 3
# 2
# 1

This is useful any time you need to count down or iterate in reverse.

Why Does Whitespace Matter in Python Loops?

The body of a for loop must be indented, otherwise you'll get a SyntaxError. Every line in the body needs to be indented the same way — the convention is 4 spaces. Pressing Tab should insert 4 spaces automatically in most editors.

This is the same indentation rule that applies to if statements, functions, and every other block in Python. Whitespace matters.

How Do While Loops Work in Python?

A while loop keeps running as long as its condition is True:

while 1:
    print("1 evaluates to True")
# prints forever (infinite loop)

That example runs forever. Normally, a while loop's condition involves a comparison that eventually becomes False:

num = 0
while num < 3:
    num += 1
    print(num)
# 1
# 2
# 3

The loop stops when num reaches 3 because 3 < 3 is False. Use a while loop when you don't know ahead of time how many iterations you'll need — for example, reading input until a user types "quit", or processing data until a counter hits a limit.

Changing a Loop With break and continue

Python provides two statements for changing a loop early: continue skips the rest of the current iteration, while break exits the loop entirely. The examples and common mistakes are covered in the focused guide to break vs continue in Python.