Counter

from collections import Counter

Count Str Characters

my_counter = Counter("Leetcode 4 life!")

Elements

sorted(my_counter.elements())
[' ', ' ', '!', '4', 'L', 'c', 'd', 'e', 'e', 'e', 'e', 'f', 'i', 'l', 'o', 't']

Most Common

Counter('abracadabra').most_common(3)
[('a', 5), ('b', 2), ('r', 2)]

Common Patterns

sum(c.values())                 # total of all counts
c.clear()                       # reset all counts
list(c)                         # list unique elements
set(c)                          # convert to a set
dict(c)                         # convert to a regular dictionary
c.items()                       # convert to a list of (elem, cnt) pairs
Counter(dict(list_of_pairs))    # convert from a list of (elem, cnt) pairs
c.most_common()[:-n-1:-1]       # n least common elements
+c                              # remove zero and negative counts

Math Patterns

c = Counter(a=3, b=1)
d = Counter(a=1, b=2)
c + d                       # add two counters together:  c[x] + d[x]
Counter({'a': 4, 'b': 3})
c - d                       # subtract (keeping only positive counts)
Counter({'a': 2})
c & d                       # intersection:  min(c[x], d[x]) 
Counter({'a': 1, 'b': 1})
c | d                       # union:  max(c[x], d[x])
Counter({'a': 3, 'b': 2})

Unary

c = Counter(a=2, b=-4)
+c
Counter({'a': 2})
-c
Counter({'b': 4})