🌱 Junior Track 17 of 18

17. Iterators & Iterables

Understand how Python traverses data behind the scenes: master the Iteration Protocol, `iter()`, `next()`, `StopIteration`, and building custom Iterator classes.

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🎓 Junior Python Curriculum — Select Module

1️⃣ Fundamentals 2️⃣ Data Types 3️⃣ Operators 4️⃣ Conditionals 5️⃣ Loops 6️⃣ Collections 7️⃣ Comprehensions 8️⃣ Functions 9️⃣ *args & **kwargs 🔟 Scope & Namespaces 1️⃣1️⃣ OOP 1️⃣2️⃣ Exceptions 1️⃣3️⃣ File Handling 1️⃣4️⃣ Modules & Packages 1️⃣5️⃣ Virtual Environments 1️⃣6️⃣ Standard Library 1️⃣7️⃣ Iterators & Iterables 1️⃣8️⃣ Generators 1️⃣9️⃣ Decorators 2️⃣0️⃣ Context Managers 2️⃣1️⃣ Lambda Functions 2️⃣2️⃣ Functional Programming 2️⃣3️⃣ Type Hints 2️⃣4️⃣ Dataclasses 2️⃣5️⃣ Testing 2️⃣6️⃣ Debugging 2️⃣7️⃣ Logging 2️⃣8️⃣ JSON & Data 2️⃣9️⃣ Regular Expressions 3️⃣0️⃣ Databases

# ==============================================================================
# JUNIOR PYTHON TRACK 17 — ITERATORS AND ITERABLES
# ==============================================================================

# 1. Understanding Iterable vs Iterator with iter() and next()
numbers = [10, 20, 30]  # 'numbers' is an Iterable

# Obtain an Iterator object from the Iterable using iter()
iterator = iter(numbers)

print(f"First element: {next(iterator)}")   # Returns 10
print(f"Second element: {next(iterator)}")  # Returns 20
print(f"Third element: {next(iterator)}")   # Returns 30

# Calling next() again raises StopIteration exception
try:
    next(iterator)
except StopIteration:
    print("Caught StopIteration: Iterator is completely exhausted!")


# 2. How Python's 'for' loop works under the hood
def custom_for_loop(iterable):
    it = iter(iterable)
    while True:
        try:
            item = next(it)
            print(f"Manual loop item: {item}")
        except StopIteration:
            break

custom_for_loop(["Python", "Flask", "PostgreSQL"])


# 3. Building a Custom Class that implements the Iterator Protocol
class Counter:
    def __init__(self, low: int, high: int):
        self.current = low
        self.high = high

    def __iter__(self):
        return self

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        val = self.current
        self.current += 1
        return val

print("Custom Counter Iterator Output:")
for count in Counter(1, 3):
    print(f"Count: {count}")

$ python3 junior_iterators.py
First element: 10
Second element: 20
Third element: 30
Caught StopIteration: Iterator is completely exhausted!
Manual loop item: Python
Manual loop item: Flask
Manual loop item: PostgreSQL
Custom Counter Iterator Output:
Count: 1
Count: 2
Count: 3
Execution completed successfully.

import unittest

class TestJuniorIterators(unittest.TestCase):
    def test_iter_next_protocol(self):
        items = ["a", "b"]
        it = iter(items)
        self.assertEqual(next(it), "a")
        self.assertEqual(next(it), "b")
        with self.assertRaises(StopIteration):
            next(it)

    def test_custom_counter(self):
        counter = Counter(5, 7)
        result = list(counter)
        self.assertEqual(result, [5, 6, 7])

if __name__ == "__main__":
    unittest.main()

🌱 Junior Track 17 Master Reference

17. Iteration Protocol Comparison Matrix

📊 Iterable vs Iterator Demystified

Feature	Iterable	Iterator
Definition	An object that can be looped over (e.g. list, dict, str).	An object representing a stream of data that delivers values one by one.
Methods Required	__iter__()	__iter__() and __next__()
Built-in Function	iter(iterable) -> iterator	next(iterator) -> value
State Tracking	Does NOT track iteration position.	Tracks current position state in memory.
Reusable?	YES (can create multiple independent iterators).	NO (exhausts once; raises StopIteration).

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17. Iterators & Iterables

🎓 Junior Python Curriculum — Select Module

⚡ Key Concept: Iterable vs. Iterator

17. Iteration Protocol Comparison Matrix

📊 Iterable vs Iterator Demystified

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