Python Generators Introduction

A Generator in Python is a special type of function that yields values one at a time, instead of returning them all at once.

Generators are memory-efficient and used for working with large datasets, streams, or infinite sequences.

Why Generators?

Feature	Why it matters
Lazy evaluation	Values are produced on demand, not all at once
Memory efficient	Doesn’t store entire data in memory
Improves performance	Faster for large datasets / loops
Infinite sequences	Possible to generate endless values

Regular Function vs Generator

Normal Function

def numbers():
    return [1,2,3]

Generator Function

def numbers():
    yield 1
    yield 2
    yield 3

The keyword yield makes it a generator.

How to Use a Generator

def numbers():
    yield 1
    yield 2
    yield 3

gen = numbers()
print(next(gen))  # 1
print(next(gen))  # 2
print(next(gen))  # 3

Generator with Loop

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

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

Difference Between return and yield

Keyword	Behavior
return	Exits function and sends a single value
yield	Pauses function, saves state, returns value, resumes on next call

Generator Expression

List comprehension

squares = [x*x for x in range(5)]

Generator expression

squares = (x*x for x in range(5))
print(squares)         # <generator object>
print(next(squares))   # 0, then 1, 4, 9...

Practical Examples

Generate Even Numbers

def even_numbers(limit):
    for n in range(limit+1):
        if n % 2 == 0:
            yield n

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

Read Large Files Line by Line (Memory Friendly)

def read_file(filepath):
    with open(filepath) as f:
        for line in f:
            yield line

for line in read_file("large_data.txt"):
    print(line)

Infinite Generator (Be careful!)

def infinite_counter():
    num = 1
    while True:
        yield num
        num += 1

gen = infinite_counter()
print(next(gen))
print(next(gen))

Fibonacci Series Generator

def fibonacci(n):
    a, b = 0, 1
    for _ in range(n):
        yield a
        a, b = b, a + b

for num in fibonacci(10):
    print(num)

Generator to Filter Data

def filter_positive(numbers):
    for num in numbers:
        if num > 0:
            yield num

nums = [-4, 3, 5, -1, 9]
for n in filter_positive(nums):
    print(n)