Container Datatypes ใน Python (โมดูล collections)

โมดูล collections ให้ Container Datatypes ที่ทรงพลังกว่า Built-ins (dict, list, set, tuple)

Container Types ภาพรวม

Class	คำอธิบาย	ใช้เมื่อไหร่
`namedtuple`	Tuple ที่มีชื่อ Field	Immutable Record, Lightweight Object
`deque`	Double-Ended Queue O(1)	Queue, Stack, Sliding Window, BFS
`Counter`	Dict Subclass นับ Hashable Objects	Frequency Counting, Multiset
`OrderedDict`	Dict จดจำ Insertion Order	LRU Cache, Ordered Config
`ChainMap`	รวมหลาย Dict เป็น View เดียว	Config Layers, Scope Chain
`defaultdict`	Dict ที่มี Default Factory	Grouping, Counting, Nested Dict
`UserDict/UserList/UserString`	Wrapper สำหรับ Subclassing	Custom Dict/List Behavior

Deque (Double-Ended Queue)

from collections import deque

dq = deque('124dfre')
dq.append('x')        # Right
dq.appendleft('B')    # Left
dq.pop()              # Right
dq.popleft()          # Left
dq.rotate(3)          # Rotate Right 3
dq.reverse()          # Reverse In-place

Methods: append, appendleft, pop, popleft, extend, extendleft, rotate, reverse, index, count, insert, remove, clear

Counter

from collections import Counter

cnt = Counter(['ABC', 'PQR', 'ABC', 'ABC', 'PQR', 'Mno', 'xyz', 'PQR', 'ABC', 'xyz'])
print(cnt)                    # Counter({'ABC': 4, 'PQR': 3, 'xyz': 2, 'Mno': 1})
print(cnt.most_common(2))     # [('ABC', 4), ('PQR', 3)]
print(list(cnt.elements()))   # ['ABC', 'ABC', 'ABC', 'ABC', 'PQR', ...]

# Arithmetic
c1 = Counter(a=3, b=1)
c2 = Counter(a=1, b=2)
print(c1 + c2)   # Counter({'a': 4, 'b': 3})
print(c1 - c2)   # Counter({'a': 2})  (เฉพาะ Positive)
print(c1 & c2)   # Intersection (Min)
print(c1 | c2)   # Union (Max)

ChainMap

from collections import ChainMap

defaults = {'host': 'localhost', 'port': 8080}
user_config = {'port': 3000}
env_config = {'host': '0.0.0.0'}

config = ChainMap(env_config, user_config, defaults)
print(config['host'])  # '0.0.0.0'
print(config['port'])  # 3000

config = config.new_child({'debug': True})  # Add Layer

OrderedDict

from collections import OrderedDict

od = OrderedDict()
od['a'] = 1
od['b'] = 2
od.move_to_end('a')     # Move to End
od.move_to_end('b', last=False)  # Move to Front
od.popitem(last=False)  # Pop Front (FIFO)

defaultdict

from collections import defaultdict

# Grouping
words = ['apple', 'bat', 'bar', 'atom', 'book']
groups = defaultdict(list)
for w in words:
    groups[w[0]].append(w)
# {'a': ['apple', 'atom'], 'b': ['bat', 'bar', 'book']}

# Nested Defaultdict
tree = lambda: defaultdict(tree)
data = tree()
data['a']['b']['c'] = 1

UserDict / UserList / UserString

from collections import UserDict

class ValidatedDict(UserDict):
    def __setitem__(self, key, value):
        if not isinstance(key, str):
            raise TypeError("Keys must be strings")
        super().__setitem__(key, value)

vd = ValidatedDict()
vd['name'] = 'John'  # OK
# vd[123] = 'Error'  # TypeError

สรุปเลือกใช้

ต้องการ Queue/Stack -> deque
นับ Frequency -> Counter
Config Layers -> ChainMap
Dict ที่ต้องการ Order -> OrderedDict (หรือ dict ใน 3.7+)
Dict ที่ Auto-create Default -> defaultdict
Custom Container Behavior -> UserDict/UserList