^^Python. collections.defaultdict(default_factory)

docs.python.org/3.8/library/collections.html

class collections.defaultdict([default_factory[, ...]])

defaultdict(list)    builds a dictionary of lists

list of key-value pairs  TO  dictionary of lists

[('c', 6), ('b', 2), ('c', 3), ('b', 4), ('a', 1), ('b', 2)]

to

{'c': [6, 3], 'b': [2, 4, 2], 'a': [1]}

from collections import defaultdict

s = [('c', 6), ('b', 2), ('c', 3), ('b', 4), ('a', 1), ('b', 2)]

d = defaultdict(list)

for k, v in s:

    d[k].append(v)

d

sorted(d.items())

When a key is

encountered the first time, it is not already in the mapping.

• default_factory function returns the default value: an empty list
• list.append() operation append the value to the new list.

encountered again, it is already in the mapping

• the look-up proceeds normally (returning the list for that key)
• list.append() operation append another value to the list.

This technique is simpler and faster than an equivalent technique using dict.setdefault().

vo:

• defaultdict(list) bilds a dictionary of lists
• Setting the default_factory to list, makes the defaultdict useful for building a dictionary of lists

defaultdict(set) builds a dictionary of sets

list of key-value pairs TO dictionary of sets

[('c', 6), ('b', 2), ('c', 3), ('b', 4), ('a', 1), ('b', 2)]

to

{'c': {3, 6}, 'b': {2, 4}, 'a': {1}}

from collections import defaultdict

s = [('c', 6), ('b', 2), ('c', 3), ('b', 4), ('a', 1), ('b', 2)]
d = defaultdict(set) 
for k, v in s: 
    d[k].add(v)

d
sorted(d.items())

defaultdict(int) builds dictionary of int.

Useful for counting, like a bag or multiset in other languages.

string TO dictionary of int

'roberto antonio occa'

to

{'r': 2, 'o': 5, 'b': 1, 'e': 1, 't': 2, ' ': 2, 'a': 2, 'n': 2, 'i': 1, 'c': 2}

from collections import defaultdict

s = 'roberto antonio occa' 

d = defaultdict(int) 

for k in s: 

    d[k] += 1

d

sorted(d.items())

{'r': 2, 'o': 5, 'b': 1, 'e': 1, 't': 2, ' ': 2, 'a': 2, 'n': 2, 'i': 1, 'c': 2}

[(' ', 2), ('a', 2), ('b', 1), ('c', 2), ('e', 1), ('i', 1), ('n', 2), ('o', 5), ('r', 2), ('t', 2)]

When a letter is

encountered the first time, it is not already in the mapping

• default_factory function returns the default value: calls int() which return zero.
• The increment operation then builds up the count for each letter.

The function int() which always returns zero is just a special case of constant functions.

Counter objects

A counter tool is provided to support convenient and rapid tallies. For example:
>>>

>>> # Tally occurrences of words in a list
>>> cnt = Counter()
>>> for word in ['red', 'blue', 'red', 'green', 'blue', 'blue']:
... cnt[word] += 1
>>> cnt
Counter({'blue': 3, 'red': 2, 'green': 1})

Find the most common words in a text

from collections import Counter

import re
words = re.findall(r'\w+', open('collodi_pinocchio.txt').read().lower())
Counter(words).most_common(20)

Regular expression. Modulo re.py
https://docs.python.org/3.8/library/re.html