Iterators और Generators

marks = [78, 91, 66]

it = iter(marks)        # ask the list for an iterator
print(next(it))
print(next(it))
print(next(it))
try:
    next(it)            # nothing left
except StopIteration:
    print("StopIteration – the loop ends here")

class Countdown:
    def __init__(self, start):
        self.current = start

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        value = self.current
        self.current -= 1
        return value

for n in Countdown(3):
    print(n, end=" ")
print("🚀")

def countdown(start):
    print("generator started")
    while start > 0:
        yield start          # pause here and hand out a value
        start -= 1
    print("generator finished")

gen = countdown(3)
print(type(gen).__name__)    # nothing has run yet
print(next(gen))
print(next(gen))
print(list(gen))             # takes the rest

import sys

squares_list = [n * n for n in range(1_000_000)]
squares_gen = (n * n for n in range(1_000_000))     # round brackets

print("list size :", sys.getsizeof(squares_list) // 1024, "KB")
print("gen size  :", sys.getsizeof(squares_gen), "bytes")
print("same total:", sum(squares_list) == sum(squares_gen))

def read_lines(path):
    with open(path, encoding="utf-8") as f:
        for line in f:                 # files are iterators too
            yield line.rstrip("\n")

def only_errors(lines):
    for line in lines:
        if " ERROR " in line:
            yield line

def parse(lines):
    for line in lines:
        date, time, level, *message = line.split()
        yield {"time": time, "message": " ".join(message)}

errors = parse(only_errors(read_lines("server.log")))
for e in errors:
    print(e["time"], "-", e["message"])

import itertools

def invoice_numbers(prefix="INV"):
    n = 1
    while True:                    # never ends – that is fine for a generator
        yield f"{prefix}-{n:04d}"
        n += 1

numbers = invoice_numbers()
print(next(numbers), next(numbers))

# take only what you need with islice
print(list(itertools.islice(invoice_numbers("SS"), 3)))

# batches of 2 students (itertools.batched exists from Python 3.12; this works everywhere)
def batches(items, size):
    it = iter(items)
    while chunk := list(itertools.islice(it, size)):
        yield chunk

print(list(batches(["Asha", "Ravi", "Neha", "Kabir", "Isha"], 2)))

def walk(folder):
    """Yield every file name inside nested folders (a dict tree)."""
    for name, value in folder.items():
        if isinstance(value, dict):
            yield from walk(value)       # hand over to the inner generator
        else:
            yield name

project = {"index.html": 1, "css": {"style.css": 1}, "js": {"app.js": 1, "lib": {"chart.js": 1}}}
print(list(walk(project)))