python collections 101

The collections module in Python provides specialized container datatypes, such as:

• Counter
• deque
• defaultdict
• namedtuple
• OrderedDict Those datatypes extend the functionality of built-in data structures.

  Counter

Description

Counter is a dictionary subclass designed for counting hashable objects. It maps elements to their counts.

When to Use

Use Counter instead of a dictionary when you need:

• To count occurrences of items in an iterable.
• To perform arithmetic and set operations on counts.

Examples

Basic Usage

from collections import Counter

c = Counter("hello world")
print(c)  # Output: Counter({'l': 3, 'o': 2, 'h': 1, 'e': 1, ' ': 1, 'w': 1, 'r': 1, 'd': 1})

Arithmetic Operations

c1 = Counter("hello")
c2 = Counter("world")
print(c1 + c2)  # Combined counts
print(c1 - c2)  # Subtracted counts

namedtuple

namedtuple is a factory function in the collections module that creates tuple subclasses with named fields. It is an immutable data structure that allows access to elements using field names instead of positional indices.

When to Use

Use namedtuple instead of regular tuples when you need:

• To represent structured data with named fields for better readability.
• Immutable objects where field names improve code clarity.
• Lightweight alternatives to full-fledged classes.

Examples

Basic Usage

from collections import namedtuple

Point = namedtuple('Point', ['x', 'y'])
p = Point(10, 20)
print(p.x, p.y)  # Output: 10 20

Advanced Features

p = p._replace(x=15)  # Replace a value
print(p._asdict())    # Convert to dictionary: {'x': 15, 'y': 20}

deque

Description

deque (double-ended queue) is a list-like container with fast appends and pops from both ends. It is implemented as a doubly-linked list.

When to Use

Use deque instead of a list when you need:

• Fast append and pop operations from both ends.
• To implement a queue, stack, or circular buffer.
• Efficient rotations or operations requiring manipulation at both ends.

Examples

Basic Usage

from collections import deque

d = deque([1, 2, 3])
d.append(4)       # Append to the right
d.appendleft(0)   # Append to the left
print(d)          # Output: deque([0, 1, 2, 3, 4])

d.pop()           # Remove from the right
d.popleft()       # Remove from the left
print(d)          # Output: deque([1, 2, 3])

Advanced Features

d = deque([1, 2, 3, 4])
d.rotate(1)  # Rotate to the right
print(d)     # Output: deque([4, 1, 2, 3])

defaultdict

Description

defaultdict is a dictionary subclass that provides a default value for non-existent keys.

When to Use

Use defaultdict instead of a dictionary when you need:

• To avoid key errors by providing default values.
• Simplify code where keys are dynamically added.

Examples

Basic Usage

from collections import defaultdict

d = defaultdict(int)  # Default value of 0
d['a'] += 1
print(d)  # Output: defaultdict(<class 'int'>, {'a': 1})

Using Lists as Default Values

d = defaultdict(list)
d['a'].append(1)
print(d)  # Output: defaultdict(<class 'list'>, {'a': [1]})