Master Python Iterators & Generators: List Comprehensions, Memory‑Efficient Loops, and Fibonacci

Python Iterators and Generators

List Comprehensions

List comprehensions (also called list comprehensions) produce a list based on a defined rule.

list2 = [x for x in range(10)]
print(list2)

A more complex example:

list1 = [x * y for x in range(1, 5) if x > 2 for y in range(1, 4) if y < 3]

# Equivalent expanded code
list4 = []
for x in range(1, 5):
    if x > 2:
        for y in range(1, 4):
            if y < 3:
                list4.append(x * y)

Generators

Creating a list with a million elements consumes a lot of memory; a generator stores only the rule and produces elements on demand, reducing memory usage. Generators can only be iterated sequentially and cannot be indexed or rewound.

To create a generator, replace the brackets with parentheses: generator = (x for x in range(10)) A generator is a special iterator:

print("Is generator an iterator:", isinstance(generator, Iterator))  # True
print("Is generator iterable:", isinstance(generator, Iterable))    # True

Elements are retrieved with next() (or generator.__next__()) and raise StopIteration when exhausted.

Generator example for the Fibonacci sequence:

def fib(length):
    """Generator that yields a Fibonacci sequence up to the given length."""
    n, a, b = 0, 0, 1
    while n < length:
        yield b
        a, b = b, a + b
        n += 1
    return 'done'

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

Iterators

An iterator is an object that implements the __next__() method and can be advanced with next(). Built‑in collections like list, dict, and str are iterable but not iterators. Converting an iterable to an iterator with iter() yields an object that is both an iterator and iterable.

list1 = [1, 2, 3, 4, 5, 6]
print("list1 is iterator:", isinstance(list1, Iterator))   # False
print("list1 is iterable:", isinstance(list1, Iterable))   # True

interObj = iter(list1)
print("interObj is iterator:", isinstance(interObj, Iterator))  # True
print("interObj is iterable:", isinstance(interObj, Iterable))  # False

Iterable objects are not necessarily iterators, but they are always iterable.

Iterators are always iterable; generators are a special kind of iterator.

The main difference is that iterables can report their length with len(), while iterators cannot.

Iterators provide a next() method to retrieve the next element; not all iterables have this method.