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"""

ITERATOR PATTERN - Python's Built-in Iterator Protocol

Python has built-in support for the Iterator Pattern using:

- __iter__() method: returns the iterator object

- __next__() method: returns the next item

- StopIteration exception: signals end of iteration

This allows using 'for' loops and other Python iteration features!

"""

# ============================================================

# PYTHONIC ITERATOR - Basic Example

# ============================================================

class Countdown:

"""Iterator that counts down from a number"""

def __init__(self, start):

self.current = start

def __iter__(self):

"""Returns the iterator object (self)"""

return self

def __next__(self):

"""Returns the next item"""

if self.current <= 0:

raise StopIteration

self.current -= 1

return self.current + 1

# ============================================================

# SEPARATE ITERATOR AND COLLECTION

# ============================================================

class NumberCollection:

"""Collection that stores numbers"""

def __init__(self, numbers):

self._numbers = list(numbers)

def __iter__(self):

"""Returns a new iterator"""

return NumberIterator(self._numbers)

def __len__(self):

return len(self._numbers)

class NumberIterator:

"""Iterator for NumberCollection"""

def __init__(self, numbers):

self._numbers = numbers

self._index = 0

def __iter__(self):

return self

def __next__(self):

if self._index >= len(self._numbers):

raise StopIteration

value = self._numbers[self._index]

self._index += 1

return value

# ============================================================

# CUSTOM ITERATORS WITH FILTERS

# ============================================================

class EvenNumbersIterator:

"""Iterator that only returns even numbers"""

def __init__(self, numbers):

self._numbers = numbers

self._index = 0

def __iter__(self):

return self

def __next__(self):

while self._index < len(self._numbers):

value = self._numbers[self._index]

self._index += 1

if value % 2 == 0:

return value

raise StopIteration

class RangeIterator:

"""Custom range iterator"""

def __init__(self, start, end, step=1):

self.current = start

self.end = end

self.step = step

def __iter__(self):

return self

def __next__(self):

if (self.step > 0 and self.current >= self.end) or \

(self.step < 0 and self.current <= self.end):

raise StopIteration

value = self.current

self.current += self.step

return value

# ============================================================

# INFINITE ITERATOR

# ============================================================

class FibonacciIterator:

"""Infinite Fibonacci sequence iterator"""

def __init__(self):

self.a = 0

self.b = 1

def __iter__(self):

return self

def __next__(self):

result = self.a

self.a, self.b = self.b, self.a + self.b

return result

# ============================================================

# COLLECTION WITH MULTIPLE ITERATION METHODS

# ============================================================

class Playlist:

"""Music playlist with different iteration modes"""

def __init__(self):

self._songs = []

def add_song(self, song):

self._songs.append(song)

def __iter__(self):

"""Default: forward iteration"""

return iter(self._songs)

def reverse_iter(self):

"""Iterate in reverse"""

return reversed(self._songs)

def random_iter(self):

"""Iterate in random order"""

import random

shuffled = self._songs.copy()

random.shuffle(shuffled)

return iter(shuffled)

# ============================================================

# USING GENERATORS (Pythonic way!)

# ============================================================

def countdown_generator(start):

"""Generator function - more Pythonic than iterator class"""

while start > 0:

yield start

start -= 1

def fibonacci_generator(limit=None):

"""Fibonacci generator with optional limit"""

a, b = 0, 1

count = 0

while limit is None or count < limit:

yield a

a, b = b, a + b

count += 1

def even_numbers_generator(numbers):

"""Generator that yields only even numbers"""

for num in numbers:

if num % 2 == 0:

yield num

# ============================================================

# CLIENT CODE

# ============================================================

if __name__ == "__main__":

print("="*60)

print(" Iterator Pattern - Python's Built-in Features")

print("="*60)

# Example 1: Basic iterator

print("\n1. BASIC ITERATOR CLASS")

print("-" * 60)

print("Countdown from 5:")

for num in Countdown(5):

print(f" {num}...", end=" ")

print("Blast off! 🚀\n")

# Example 2: Collection with iterator

print("\n2. COLLECTION WITH ITERATOR")

print("-" * 60)

numbers = NumberCollection([1, 2, 3, 4, 5])

print(f"Collection has {len(numbers)} numbers:")

for num in numbers:

print(f" {num}", end=" ")

print()

# Multiple iterations

print("\nMultiple iterations (creates new iterator each time):")

iter1 = iter(numbers)

iter2 = iter(numbers)

print(f" Iterator 1 - first: {next(iter1)}, second: {next(iter1)}")

print(f" Iterator 2 - first: {next(iter2)}, second: {next(iter2)}")

# Example 3: Filtered iterator

print("\n\n3. FILTERED ITERATOR (Even numbers only)")

print("-" * 60)

data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

print(f"Original: {data}")

print("Even numbers:", end=" ")

for num in EvenNumbersIterator(data):

print(num, end=" ")

print()

# Example 4: Custom range

print("\n\n4. CUSTOM RANGE ITERATOR")

print("-" * 60)

print("Range(0, 10, 2):", end=" ")

for num in RangeIterator(0, 10, 2):

print(num, end=" ")

print()

print("Range(10, 0, -1):", end=" ")

for num in RangeIterator(10, 0, -1):

print(num, end=" ")

print()

# Example 5: Infinite iterator (with limit)

print("\n\n5. INFINITE ITERATOR (Fibonacci)")

print("-" * 60)

print("First 10 Fibonacci numbers:")

fib = FibonacciIterator()

for i in range(10):

print(f" F({i}) = {next(fib)}")

# Example 6: Multiple iteration modes

print("\n\n6. MULTIPLE ITERATION MODES (Playlist)")

print("-" * 60)

playlist = Playlist()

for song in ["Song A", "Song B", "Song C", "Song D"]:

playlist.add_song(song)

print("Forward:")

print(" " + " → ".join(playlist))

print("\nReverse:")

print(" " + " → ".join(playlist.reverse_iter()))

print("\nRandom (shuffle):")

print(" " + " → ".join(playlist.random_iter()))

# Example 7: Generators (Pythonic way!)

print("\n\n7. GENERATORS (More Pythonic)")

print("-" * 60)

print("Countdown generator:")

for num in countdown_generator(5):

print(f" {num}...", end=" ")

print("Done!")

print("\n\nFibonacci generator (first 10):")

for num in fibonacci_generator(10):

print(f" {num}", end=" ")

print()

print("\n\nEven numbers generator:")

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

evens = list(even_numbers_generator(numbers))

print(f" {evens}")

# Example 8: Built-in Python iteration tools

print("\n\n8. PYTHON'S BUILT-IN ITERATION TOOLS")

print("-" * 60)

data = [1, 2, 3, 4, 5]

print(f"Original: {data}")

print(f"Reversed: {list(reversed(data))}")

print(f"Enumerate: {list(enumerate(data))}")

print(f"Zip: {list(zip(data, ['a', 'b', 'c', 'd', 'e']))}")

# Example 9: Iterator protocol in action

print("\n\n9. ITERATOR PROTOCOL IN ACTION")

print("-" * 60)

print("Manual iteration (what 'for' loop does internally):")

collection = NumberCollection([10, 20, 30])

iterator = iter(collection) # Calls __iter__()

try:

while True:

item = next(iterator) # Calls __next__()

print(f" Got: {item}")

except StopIteration:

print(" Iteration complete!")

print("\n" + "="*60)

print("🔑 PYTHON'S ITERATOR FEATURES:")

print("="*60)

print("""

1. PROTOCOL METHODS:

__iter__(): Returns iterator object

__next__(): Returns next item or raises StopIteration

2. FOR LOOP SUPPORT:

for item in collection:

# Automatically uses __iter__() and __next__()

3. BUILT-IN FUNCTIONS:

iter() - Get iterator from iterable

next() - Get next item from iterator

reversed() - Reverse iteration

enumerate() - Add index to iteration

zip() - Parallel iteration

4. GENERATORS (Recommended):

def my_gen():

yield item

# Simpler than creating iterator classes!

5. COMPREHENSIONS:

[x for x in collection] # List comprehension

(x for x in collection) # Generator expression

{x for x in collection} # Set comprehension

{k:v for k,v in items} # Dict comprehension

""")

print("\n" + "="*60)

print("💡 BEST PRACTICE:")

print("="*60)

print("""

In Python, prefer GENERATORS over ITERATOR CLASSES:

❌ Old way (verbose):

class MyIterator:

def __init__(self, data):

self.data = data

self.index = 0

def __iter__(self):

return self

def __next__(self):

if self.index >= len(self.data):

raise StopIteration

result = self.data[self.index]

self.index += 1

return result

✅ New way (Pythonic):

def my_generator(data):

for item in data:

yield item

Generators are simpler, more readable, and memory-efficient!

""")