Loops in Python

If you’re new to Python, or perhaps coding in general (welcome!), loops are what allow us to do the same thing over and over and over again, without having to re-type the same code each time. For example, let’s pretend I want to print the numbers 0-9.

I could do this:

print(0)
print(1)
print(2)
print(3)
print(4)
print(5)
print(6)
print(7)
print(8)
print(9)

However, it would save me a lot of time typing to use a loop. Especially if I wanted to do the same thing one thousand or one million times.

A “for loop” in Python is written like this:

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

i replaces the numbers 0 to 9. In English, the code says:

1. Start with i equals 0. (i in range(0))
2. If i is not less than 10 (range(0, 10)), exit the loop. Else:
   1. Print i to the console. (print(i))
   2. Add 1 to i. (range defaults to incrementing by 1)
   3. Go back to step 2.

The result is that the numbers 0-9 are logged to the console in order.

We’re making all the static content from our Boot.dev courses available for free here on the blog. This one is the “Loops” chapter from Learn to Code in Python. If you want to try the far more interactive version of the course, do check it out!

By the way, whitespace matters in Python!

The body of a for-loop must be indented; otherwise, you’ll get a syntax error. Every line in the body of the loop must be indented in the same way - we use the “4 spaces” convention. Pressing the <tab> key should automatically insert 4 spaces.

Range Continued 🔗

The range() function we’ve been using in our for loops actually has an optional 3rd parameter: the “step”.

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

The “step” parameter determines how much to add to i in each iteration of the loop. You can even go backwards:

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

Iterating over lists 🔗

While explictly looping from one number to another is useful, more often than not you’ll probably want to iterate over all the elements in a list. In my opinion, Python has the most elegant syntax for iterating directly over the items in a list without worrying about index numbers. If you don’t need the index number you can use the following syntax:

trees = ['oak', 'pine', 'maple']
for tree in trees:
    print(tree)
# Prints:
# oak
# pine
# maple

tree, the variable declared using the in keyword, directly accesses the value in the list rather than the index of the value. If we don’t need to update the item and only need to access its value then this is a more clean way to write the code.

While 🔗

Python has another type of loop, the while loop. It’s a loop that continues while a condition remains True. The syntax is simple:

while 1:
    print("1 evaluates to True")

# prints:
# 1 evaluates to True
# 1 evaluates to True
# (...continuing)

The example above is hardcoded to continue forever, creating an infinite loop. Typically, a while loop’s condition will have an explicit comparison that determines when the loop ends:

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

# prints:
# 1
# 2
# 3
# (the loop stops when num >= 3)

With loops, we can efficiently automate repetitive tasks, manage data sequences, and streamline complex processes in programming. Mastering loops empowers us to write cleaner, more efficient code, and to embrace the true power of programming automation.