Python Generators & Iterators Cheat Sheet

Lazy iteration in Python covering generator functions, yield from, the iterator protocol, and commonly used itertools functions.

1 PageIntermediateApr 15, 2026

Generator Functions (yield)

Functions that produce a lazy sequence of values.

python

def count_up_to(n):    i = 1    while i <= n:        yield i        i += 1for num in count_up_to(5):    print(num)   # 1 2 3 4 5gen = count_up_to(3)next(gen)   # 1next(gen)   # 2

Generator Expressions

A compact, lazily-evaluated alternative to list comprehensions.

python

squares = (x ** 2 for x in range(10))next(squares)   # 0list(squares)    # remaining values: [1, 4, 9, ..., 81]# memory-efficient aggregation, no intermediate list builttotal = sum(x ** 2 for x in range(1_000_000))

yield from & Delegation

Delegating iteration to a sub-generator or iterable.

python

def chain(*iterables):    for it in iterables:        yield from itlist(chain([1, 2], [3, 4], (5, 6)))# [1, 2, 3, 4, 5, 6]def flatten(nested):    for item in nested:        if isinstance(item, list):            yield from flatten(item)        else:            yield item

The Iterator Protocol (iter/next)

Building a custom class that supports the for loop.

python

class Counter:    def __init__(self, limit):        self.limit = limit        self.n = 0    def __iter__(self):        return self    def __next__(self):        if self.n >= self.limit:            raise StopIteration        self.n += 1        return self.nfor x in Counter(3):    print(x)   # 1 2 3

Useful itertools Functions

Standard-library building blocks for iterator composition.

count(start=0, step=1)- infinite arithmetic sequence, e.g. count(10) -> 10, 11, 12, ...
cycle(iterable)- repeats the iterable indefinitely
chain(*iterables)- iterates through multiple iterables as one sequence
islice(iterable, stop)- slices an iterator lazily, like list slicing
zip_longest(a, b)- zips iterables, filling missing values with fillvalue
groupby(iterable, key)- groups consecutive elements sharing a key
product(a, b)- Cartesian product, equivalent to nested for loops

Pro Tip

A generator can only be iterated once — once exhausted (or partially consumed with next()), it cannot be reset. If you need to iterate the same data multiple times, convert it to a list or re-create the generator.

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