Day 17 of 30: 🧮 List Comprehensions & Generators

🔄 Quick Recap (Day 16)

You learned to isolate projects with virtual environments.
You managed dependencies with pip and froze requirements for sharing.

🎯 What You’ll Learn Today

What a list comprehension is and how it simplifies creating lists.
How to add conditions inside comprehensions.
What a generator is and why it’s memory-efficient.
How to write generator expressions and custom generator functions.

📖 List Comprehensions: Quick Lists

A list comprehension combines a loop and optional condition into a single, concise expression.

Standard loop:

squares = []
for x in range(1, 6):
    squares.append(x * x)
print(squares)

List comprehension:

squares = [x * x for x in range(1, 6)]
print(squares)

Both produce:

[1, 4, 9, 16, 25]

Adding Conditions

Include an if to filter items:

evens = [x for x in range(1, 11) if x % 2 == 0]
print(evens)  # [2, 4, 6, 8, 10]

📖 Generators: Lazy Sequences

Generators yield items one at a time, saving memory for large sequences.

Generator Expression

Similar syntax to list comprehensions but with parentheses:

big_squares = (x * x for x in range(1, 1000000))
print(next(big_squares))  # 1
print(next(big_squares))  # 4

Only computes values as requested.

Generator Function

Use yield inside a function:

def countdown(n):
    while n > 0:
        yield n
        n -= 1

for number in countdown(5):
    print(number)

Outputs:

🧙‍♂️ Take the Wand and Try Yourself

Create a file comp_gen.py.
List comprehension:
- Generate a list of the cubes of numbers 1–10.
- Filter to include only cubes divisible by 3.
Generator expression:
- Create a generator for the same cubes.
- Use next() to print the first three values.
Generator function:
- Write even_numbers(n) that yields even numbers up to n.
- Use a loop to print all evens up to 10.

Solution Example (comp_gen.py):

# comp_gen.py

# List comprehension with condition
cubes = [x**3 for x in range(1, 11) if (x**3) % 3 == 0]
print(cubes)  # [27, 216, 729]

# Generator expression
cube_gen = (x**3 for x in range(1, 11))
print(next(cube_gen))  # 1
print(next(cube_gen))  # 8
print(next(cube_gen))  # 27

# Generator function
def even_numbers(n):
    for x in range(1, n+1):
        if x % 2 == 0:
            yield x

for e in even_numbers(10):
    print(e)

Expected output:

[27, 216, 729]
1
8
27
2
4
6
8
10

Run:

python comp_gen.py

Once your output matches, you’ve mastered list comprehensions and generators!

Up next: Day 17: Standard Library Deep Dive