12_generators.md

🐍 12 – Generators & yield

🎯 What You'll Learn

• What generators are and why they save memory
• yield keyword
• Generator expressions
• When to use generators vs lists

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📌 JS vs Python — Side by Side

Concept	JavaScript	Python
Generator function	function* gen() { yield x; }	def gen(): yield x
Generator expression	no equivalent	(x*2 for x in range(10))
next()	gen.next().value	next(gen)

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💡 The Problem Generators Solve

# BAD — loads ALL 1 million numbers into memory at once
def get_million():
    return [i for i in range(1_000_000)]

# GOOD — generates ONE number at a time, uses almost no memory
def get_million():
    for i in range(1_000_000):
        yield i

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💡 Basic Generator

def countdown(n):
    while n > 0:
        yield n     # pause here, return n, resume next call
        n -= 1

# Use in a for loop
for num in countdown(5):
    print(num)
# 5 4 3 2 1

# Or manually with next()
gen = countdown(3)
print(next(gen))    # 3
print(next(gen))    # 2
print(next(gen))    # 1
print(next(gen))    # StopIteration error

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💡 Generator Expressions

Like list comprehensions but lazy — no parentheses [], use ().

# List comprehension — creates full list in memory
squares_list = [x**2 for x in range(10)]

# Generator expression — computes one at a time
squares_gen  = (x**2 for x in range(10))

# Sum without building a list
total = sum(x**2 for x in range(1000))

# Very memory efficient for large datasets
with open("huge_file.txt") as f:
    long_lines = (line for line in f if len(line) > 100)
    for line in long_lines:
        process(line)

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💡 JavaScript Equivalent

function* countdown(n) {
  while (n > 0) {
    yield n;
    n--;
  }
}

const gen = countdown(5);
console.log(gen.next().value);  // 5
console.log(gen.next().value);  // 4

Python's syntax is cleaner — no function*, just regular def with yield.

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⚠️ Key Points

• Generators can only be iterated once — they are exhausted after
• Use list(gen) to convert to a list (but loses memory benefit)
• Perfect for reading large files, streaming data, infinite sequences

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🔗 Next Lesson

👉 13 – OOP Basics