Loops, lists, and the patterns you'll use daily

Why this matters

Computers are good at one thing: doing the same thing over and over without getting bored. Loops let you repeat a block of code many times, and lists let you hold many values under one name. Together they unlock nearly every real program: process every row of a spreadsheet, ask every user a question, build up a collection of results, search through items to find one that matches. This lesson is the last warm-up you need before the existing course picks up the pace.

Learning objectives

After this lesson you will be able to:

1. Create a list and read / change / add / remove items.
2. Use a for loop to visit every item in a list.
3. Build a new list with a comprehension.
4. Use range, enumerate, and zip to loop with index and pairing.
5. Loop with while when you do not know how many times in advance.

1. Lists — holding many values

A list holds an ordered collection of values, inside square brackets, separated by commas.

colors = ["red", "green", "blue"]
numbers = [1, 2, 3, 4, 5]
mixed = ["a", 1, True]
empty = []

Reading items — counting from 0

colors[0]     # 'red'
colors[1]     # 'green'
colors[-1]    # 'blue'   (negative counts from the end)
len(colors)   # 3        (how many items)

Changing items

colors[0] = "purple"
print(colors)    # ['purple', 'green', 'blue']

Adding and removing

colors.append("yellow")          # add to the end
colors.insert(0, "orange")       # add at a position
colors.remove("green")           # remove first matching item
last = colors.pop()              # remove and return the last item

Slicing — a chunk of the list

colors[1:3]    # items at index 1 and 2 (2 is not included)

2. for loops — do something for each item

The for loop visits every item in a sequence.

colors = ["red", "green", "blue"]

for color in colors:
    print(color)

Prints:

red
green
blue

Read it as: "for each color in colors, print it."

Looping over a string

A string is also a sequence of characters:

for ch in "hi":
    print(ch)     # h, then i

for over a list of numbers

total = 0
for n in [1, 2, 3, 4]:
    total = total + n
print(total)     # 10

This "accumulate a running total" pattern is everywhere.

3. range — count without making a list

range(n) produces the numbers 0 to n-1 without creating a big list in memory.

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

You can set start, stop, and step:

range(1, 4)       # 1, 2, 3
range(0, 10, 2)   # 0, 2, 4, 6, 8
range(5, 0, -1)   # 5, 4, 3, 2, 1

4. enumerate — get the index too

Sometimes you want both the item and its position. enumerate gives you both.

colors = ["red", "green", "blue"]

for index, color in enumerate(colors):
    print(index, color)

Prints:

0 red
1 green
2 blue

The index, color in the loop unpacks the pair enumerate feeds you. This is your first taste of tuple unpacking (more later).

5. zip — loop over two lists side by side

zip pairs up items from two or more lists.

names = ["Ada", "Grace", "Linus"]
roles = ["engineer", "admiral", "hacker"]

for name, role in zip(names, roles):
    print(f"{name} is a {role}")

6. List comprehensions — build a list in one line

The most "Pythonic" pattern. Instead of:

squares = []
for n in range(5):
    squares.append(n * n)
print(squares)    # [0, 1, 4, 9, 16]

You can write:

squares = [n * n for n in range(5)]
print(squares)    # [0, 1, 4, 9, 16]

Read it as: "make a list, where each item is n * n, for each n in range(5)."

You can add a condition:

evens = [n for n in range(10) if n % 2 == 0]
print(evens)     # [0, 2, 4, 6, 8]

Rule of thumb: if the comprehension fits comfortably on one line and reads clearly, use it. If it needs more than one line, use a regular for loop. Comprehensions are great; overstuffed ones are not.

7. while loops — repeat until something changes

A while loop repeats as long as its condition stays true.

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

This prints 0, 1, 2 then stops when count reaches 3.

The danger: if the condition never becomes false, the loop runs forever. Make sure something inside the loop changes the condition. (If you ever do run an infinite loop, you can press Ctrl+C to interrupt it.)

8. break and continue

• break stops the loop immediately.
• continue skips to the next iteration.

for n in [1, 2, 3, 4, 5]:
    if n == 3:
        break          # stop entirely at 3
    print(n)           # 1, 2

for n in [1, 2, 3, 4, 5]:
    if n == 3:
        continue       # skip 3, keep going
    print(n)           # 1, 2, 4, 5

Common mistakes

1. Off-by-one in range. range(3) gives 0,1,2, not 1,2,3. If you want 1,2,3 use range(1, 4).
2. Forgetting to change the loop variable in while → infinite loop.
3. Mutating a list while looping over it. Changing a list during a for loop over that same list causes surprising results. Build a new list instead.
4. Confusing append with overwriting. x = [1]; y = x then y.append(2) also changes x, because both names point to the same list. (This is that "names vs references" idea from later — flag it, don't stress.)
5. Off-by-one in indexing. colors[len(colors)] is an error — the last valid index is len(colors) - 1.

Hands-on lab (1 hour)

1. Create a list of 5 numbers. Print the sum with a for loop, then with sum(numbers).
2. Print each item in a list of names with its number using enumerate.
3. Build a list of even numbers from 0 to 20 with a comprehension.
4. Use zip to print "A pairs with 1" style lines from two lists.
5. Write a while loop that counts down from 5 to 1 and prints "Liftoff!".
6. Challenge: given a list of words, make a new list of words longer than 4 characters, using a comprehension with an if.

Self-check

1. What does range(4) produce?
2. What does a list comprehension [x * 2 for x in range(3)] give?
3. What does enumerate add to a plain for loop?
4. When would you use while instead of for?
5. What is the difference between break and continue?

References

• The Python Tutorial — Lists
• The Python Tutorial — More Control Flow: for, while, comprehensions
• Real Python — Python for Loops
• Real Python — List Comprehensions