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Python Collections Module

Deque​

Defaultdict​

  • Dictionary subclass that accepts a callable in its constructor whose return value will be used if a requested key cannot be found.
from collections import defaultdict
d = defaultdict(int)  #we can insert int(defulat-0), list(defualt-[]),set(default-new Set()/{}),

d["dog"] = d["dog"] + 1  #defualt value 0 + 1 = 1
print(d["dog"])
print(d["cat"])  #returns zero default value
from collections import defaultdict
d = defaultdict(lambda: 1) #use this syntax to provide your own custom vallue
print(d['cat'])

orderedDict​

  • Dictionary subclass that remembers the insertion order of keys added to it: collections.OrderedDict
#same as dict but keeps the order
from collections import OrderedDict
d = OrderedDict(one=1, two=2, three=3)
print(d)
d['four'] = 4
print(d)
print(d.keys())

Counter​

  • Dictionary to count several different objects at once. In this case, the keys will store individual objects, and the values will hold the number of repetitions of a given object, or the object’s count.
from collections import Counter
letters = Counter("mississippi")
print(letters)
print(letters["m"])  #the output of Counter is dictionary
print(letters.keys())

print(letters.most_common(1)) #get one most common

ChainMap​

  • The collections.ChainMap data structure groups multiple dictionaries into a single mapping.
  • Lookups search the underlying mappings one by one until a key is found.
  • Insertions, updates, and deletions only affect the first mapping added to the chain.
from collections import ChainMap
dict1 = {'one': 1, 'two': 2}
dict2 = {'three': 3, 'four': 4}
chain = ChainMap(dict1, dict2)
print(chain)
chain["five"] = 5 #insertion is only on first dictionary
print(chain)