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The collections module provides specialized container datatypes that extend or provide alternatives to Python’s built-in containers (dict, list, set, tuple).

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Module Import

from collections import Counter, defaultdict, deque, namedtuple, OrderedDict, ChainMap

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Counter - Count Hashable Objects

Dictionary subclass for counting hashable objects.

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Creating Counter

from collections import Counter

# From iterable
fruits = ['apple', 'banana', 'apple', 'orange', 'banana', 'apple']
counter = Counter(fruits)
print(counter)  # Counter({'apple': 3, 'banana': 2, 'orange': 1})

# From string
counter = Counter('abracadabra')
print(counter)  # Counter({'a': 5, 'b': 2, 'r': 2, 'c': 1, 'd': 1})

# From dictionary
counter = Counter({'red': 4, 'blue': 2})

# From keyword arguments
counter = Counter(cats=4, dogs=8)

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Counter Operations

from collections import Counter

counter = Counter(['apple', 'banana', 'apple', 'orange', 'banana', 'apple'])

# Get count
print(counter['apple'])    # 3
print(counter['grape'])    # 0 (no KeyError)

# Most common
print(counter.most_common(2))  # [('apple', 3), ('banana', 2)]

# All elements
print(list(counter.elements()))  # ['apple', 'apple', 'apple', 'banana', 'banana', 'orange']

# Update counts
counter.update(['banana', 'grape'])

# Subtract counts
counter.subtract(['apple', 'banana'])

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Counter Arithmetic

from collections import Counter

c1 = Counter(['a', 'b', 'c', 'a'])
c2 = Counter(['a', 'b', 'd'])

# Addition
print(c1 + c2)  # Counter({'a': 3, 'b': 2, 'c': 1, 'd': 1})

# Subtraction (keep only positive counts)
print(c1 - c2)  # Counter({'a': 1, 'c': 1})

# Intersection (minimum)
print(c1 & c2)  # Counter({'a': 1, 'b': 1})

# Union (maximum)
print(c1 | c2)  # Counter({'a': 2, 'b': 1, 'c': 1, 'd': 1})

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defaultdict - Dict with Default Values

Dictionary subclass that provides default values for missing keys.

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Creating defaultdict

from collections import defaultdict

# With list as default
dd = defaultdict(list)
dd['colors'].append('red')
dd['colors'].append('blue')
print(dd)  # defaultdict(<class 'list'>, {'colors': ['red', 'blue']})

# With int as default (useful for counting)
dd = defaultdict(int)
for word in ['apple', 'banana', 'apple']:
    dd[word] += 1
print(dd)  # defaultdict(<class 'int'>, {'apple': 2, 'banana': 1})

# With set as default
dd = defaultdict(set)
dd['tags'].add('python')
dd['tags'].add('coding')

# With custom default
def default_value():
    return 'N/A'

dd = defaultdict(default_value)
print(dd['missing'])  # 'N/A'

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Practical Uses

from collections import defaultdict

# Group by key
data = [('fruit', 'apple'), ('veg', 'carrot'), ('fruit', 'banana')]
grouped = defaultdict(list)
for category, item in data:
    grouped[category].append(item)
print(dict(grouped))  # {'fruit': ['apple', 'banana'], 'veg': ['carrot']}

# Count by category
words = ['apple', 'banana', 'apple', 'cherry', 'banana']
count = defaultdict(int)
for word in words:
    count[word] += 1

# Nested defaultdict
tree = lambda: defaultdict(tree)
users = tree()
users['john']['age'] = 30
users['john']['city'] = 'NYC'

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deque - Double-Ended Queue

List-like container with fast appends and pops on both ends.

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Creating deque

from collections import deque

# Empty deque
d = deque()

# From iterable
d = deque([1, 2, 3, 4, 5])

# With maximum length
d = deque([1, 2, 3], maxlen=5)

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deque Operations

from collections import deque

d = deque([1, 2, 3])

# Append to right
d.append(4)  # deque([1, 2, 3, 4])

# Append to left
d.appendleft(0)  # deque([0, 1, 2, 3, 4])

# Pop from right
right = d.pop()  # 4, deque([0, 1, 2, 3])

# Pop from left
left = d.popleft()  # 0, deque([1, 2, 3])

# Extend
d.extend([4, 5])  # deque([1, 2, 3, 4, 5])
d.extendleft([0, -1])  # deque([-1, 0, 1, 2, 3, 4, 5])

# Rotate
d.rotate(2)  # Rotate right
d.rotate(-2)  # Rotate left

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deque Use Cases

from collections import deque

# Fixed-size rolling window
window = deque(maxlen=3)
for i in range(10):
    window.append(i)
    print(list(window))  # Always max 3 items

# Queue (FIFO)
queue = deque()
queue.append('first')
queue.append('second')
item = queue.popleft()  # 'first'

# Stack (LIFO)
stack = deque()
stack.append('first')
stack.append('second')
item = stack.pop()  # 'second'

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namedtuple - Tuple with Named Fields

Factory function for creating tuple subclasses with named fields.

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Creating namedtuple

from collections import namedtuple

# Define a namedtuple class
Point = namedtuple('Point', ['x', 'y'])

# Create instances
p1 = Point(10, 20)
p2 = Point(x=5, y=15)

# Access fields
print(p1.x)  # 10
print(p1.y)  # 20

# Also works with indexing
print(p1[0])  # 10

# Unpack like regular tuple
x, y = p1

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namedtuple Features

from collections import namedtuple

Person = namedtuple('Person', ['name', 'age', 'city'])
person = Person('Alice', 30, 'NYC')

# Convert to dict
print(person._asdict())  # {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Replace fields (returns new instance)
updated = person._replace(age=31)
print(updated)  # Person(name='Alice', age=31, city='NYC')

# Get field names
print(Person._fields)  # ('name', 'age', 'city')

# Create from iterable
data = ['Bob', 25, 'LA']
person = Person._make(data)

# With defaults (Python 3.7+)
Person = namedtuple('Person', ['name', 'age', 'city'], defaults=['Unknown', 0, 'Unknown'])

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OrderedDict - Dictionary That Remembers Order

As of Python 3.7+, regular dicts maintain insertion order. OrderedDict is mainly useful for its additional methods.
from collections import OrderedDict

# Create ordered dictionary
od = OrderedDict()
od['first'] = 1
od['second'] = 2
od['third'] = 3

# Move to end
od.move_to_end('first')  # Moves 'first' to the end
od.move_to_end('third', last=False)  # Moves 'third' to the beginning

# Pop items
od.popitem(last=True)  # Pop from end (LIFO)
od.popitem(last=False)  # Pop from beginning (FIFO)

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ChainMap - Combine Multiple Dicts

Groups multiple dictionaries into a single view. 
from collections import ChainMap

# Combine dictionaries
defaults = {'color': 'red', 'user': 'guest'}
config = {'user': 'admin'}

combined = ChainMap(config, defaults)
print(combined['user'])   # 'admin' (from config)
print(combined['color'])  # 'red' (from defaults)

# Update only affects first mapping
combined['user'] = 'root'
print(config)  # {'user': 'root'}
print(defaults)  # {'color': 'red', 'user': 'guest'}

# Add new mapping
new_config = {'theme': 'dark'}
combined = combined.new_child(new_config)

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Practical Examples

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Word Frequency Analysis

from collections import Counter
import re

def analyze_text(text):
    """Analyze word frequencies in text"""
    words = re.findall(r'\w+', text.lower())
    counter = Counter(words)
    
    print(f"Total words: {len(words)}")
    print(f"Unique words: {len(counter)}")
    print("\nTop 10 most common:")
    for word, count in counter.most_common(10):
        print(f"  {word}: {count}")

# Usage
text = "Python is great. Python is powerful. Python is easy to learn."
analyze_text(text)

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LRU Cache Implementation

from collections import OrderedDict

class LRUCache:
    def __init__(self, capacity):
        self.cache = OrderedDict()
        self.capacity = capacity
    
    def get(self, key):
        if key not in self.cache:
            return None
        # Move to end (most recently used)
        self.cache.move_to_end(key)
        return self.cache[key]
    
    def put(self, key, value):
        if key in self.cache:
            self.cache.move_to_end(key)
        self.cache[key] = value
        if len(self.cache) > self.capacity:
            # Remove least recently used
            self.cache.popitem(last=False)

# Usage
cache = LRUCache(3)
cache.put('a', 1)
cache.put('b', 2)
cache.put('c', 3)
cache.put('d', 4)  # 'a' is evicted

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Group Items by Property

from collections import defaultdict

data = [
    {'name': 'Alice', 'dept': 'Engineering'},
    {'name': 'Bob', 'dept': 'Sales'},
    {'name': 'Charlie', 'dept': 'Engineering'},
]

# Group by department
by_dept = defaultdict(list)
for person in data:
    by_dept[person['dept']].append(person['name'])

print(dict(by_dept))
# {'Engineering': ['Alice', 'Charlie'], 'Sales': ['Bob']}

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Sliding Window Average

from collections import deque

def moving_average(data, window_size):
    """Calculate moving average with sliding window"""
    window = deque(maxlen=window_size)
    averages = []
    
    for value in data:
        window.append(value)
        if len(window) == window_size:
            avg = sum(window) / window_size
            averages.append(avg)
    
    return averages

# Usage
data = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
averages = moving_average(data, 3)
print(averages)  # [2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0]

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Best Practices

Use Counter for frequency analysis: Counter is optimized for counting and provides useful methods like most_common().
from collections import Counter

# Fast and readable
counts = Counter(items)

# Instead of
counts = {}
for item in items:
    counts[item] = counts.get(item, 0) + 1
Use deque for queues and stacks: deque has O(1) append and pop from both ends, unlike lists.
from collections import deque

# Efficient queue
queue = deque()
queue.append(item)  # O(1)
queue.popleft()     # O(1)

# List is slower for queues
queue = []
queue.append(item)  # O(1)
queue.pop(0)        # O(n)

Built-in Types

Standard container types

itertools

Iterator building blocks

heapq

Heap queue algorithm

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