Master Python’s collections: Counter, defaultdict, deque, and more

The collections module offers specialized container types that serve as alternatives to Python’s built‑in dict, list, set, and tuple. It includes Counter, defaultdict, OrderedDict, namedtuple, deque, and ChainMap.

Counter

Counter

is a dict subclass for counting hashable objects.

>> import collections
>>> # count characters
>>> collections.Counter('hello world')
Counter({'l': 3, 'o': 2, 'h': 1, 'e': 1, ' ': 1, 'w': 1, 'r': 1, 'd': 1})
>>> # count words
>>> collections.Counter('hello world hello lucy'.split())
Counter({'hello': 2, 'world': 1, 'lucy': 1})

Common methods: elements(): iterator over elements repeating each as many times as its count. most_common([n]): list of the n most common elements and their counts. subtract(iterable-or-mapping): decrement counts using another iterable or mapping. update(iterable-or-mapping): increment counts from another iterable or mapping.

defaultdict

defaultdict

returns a new dictionary‑like object that provides a default value for missing keys via a factory function.

>> d = collections.defaultdict()
defaultdict(None, {})
>>> e = collections.defaultdict(str)
defaultdict(<class 'str'>, {})
>>> e['hello']
''
>>> e
defaultdict(<class 'str'>, {'hello': ''})
>>> fruit = collections.defaultdict(int)
>>> fruit['apple'] = 2
>>> fruit['banana']
0
>>> fruit
defaultdict(<class 'int'>, {'apple': 2, 'banana': 0})
>>> s = [('yellow',1),('blue',2),('yellow',3),('blue',4),('red',1)]
>>> d = collections.defaultdict(list)
>>> for k,v in s:
...     d[k].append(v)
>>> d
defaultdict(<class 'list'>, {'yellow': [1, 3], 'blue': [2, 4], 'red': [1]})

OrderedDict

OrderedDict

preserves the insertion order of keys.

>> o = collections.OrderedDict()
>>> o['k1'] = 'v1'
>>> o['k3'] = 'v3'
>>> o['k2'] = 'v2'
>>> o
OrderedDict([('k1', 'v1'), ('k3', 'v3'), ('k2', 'v2')])
>>> o['k1'] = 666
>>> o
OrderedDict([('k1', 666), ('k3', 'v3'), ('k2', 'v2')])

namedtuple

namedtuple

creates tuple subclasses with named fields.

>> Person = collections.namedtuple('Person', ['name', 'age', 'height'])
>>> lucy = Person('lucy', 23, 180)
>>> lucy
Person(name='lucy', age=23, height=180)
>>> lucy.name
'lucy'
>>> lucy.age
23

deque

deque

provides a thread‑safe double‑ended queue with O(1) appends and pops from both ends.

>> d = collections.deque(maxlen=10)
>>> d.extend('python')
>>> list(d)
['p', 'y', 't', 'h', 'o', 'n']
>>> d.append('e')
>>> d.appendleft('f')
>>> d.appendleft('g')
>>> d.appendleft('h')
>>> d
deque(['h', 'g', 'f', 'p', 'y', 't', 'h', 'o', 'n', 'e'], maxlen=10)
>>> d.appendleft('i')
>>> d
deque(['i', 'h', 'g', 'f', 'p', 'y', 't', 'h', 'o', 'n'], maxlen=10)

Key methods include append, appendleft, pop, popleft, extend, extendleft, clear, rotate, and others.

ChainMap

ChainMap

groups multiple dictionaries (or mappings) into a single view for lookup, preserving the order of the mappings.

>> d1 = {'apple': 1, 'banana': 2}
>>> d2 = {'orange': 2, 'apple': 3, 'pike': 1}
>>> combined = collections.ChainMap(d1, d2)
>>> combined
ChainMap({'apple': 1, 'banana': 2}, {'orange': 2, 'apple': 3, 'pike': 1})
>>> for k, v in combined.items():
...     print(k, v)
orange 2
apple 1
pike 1
banana 2
>>> combined['apple'] = 2  # modifies the first mapping
>>> combined
ChainMap({'apple': 2, 'banana': 2}, {'orange': 2, 'apple': 3, 'pike': 1})

Modifications affect only the first mapping in the chain; new keys are added there.