[Python] Dictionary (hash)

In Python, a hash (more precisely, a hash table) is called a dictionary.

Counter and defaultdict, such as when using the dictionary function to count the number of occurrences of each KW. It is good to use a subclass of the dictionary (/ //docs.python.jp/2/library/collections.html#collections.defaultdict). For example, if Counter and defaultdict are not used, redundant description is required as shown below.

`sample1.py`


# -*- coding: UTF-8 -*-

#Dictionary initialization
count_by_kw = {}

#KeyError
count_by_kw['apple'] += 1

#It's verbose, but you need to write:
if count_by_kw.get('apple'):
    count_by_kw['apple'] += 1
else:
    count_by_kw['apple'] = 1

If you use Counter, you can simply describe this as follows. You can also use the most_common method to retrieve keys in descending order of value.

`sample2.py`


# -*- coding: UTF-8 -*-
from collections import Counter

#Dictionary initialization
count_by_kw = Counter()

#Can be a simple description
count_by_kw['apple'] += 1
count_by_kw['apple'] += 1
count_by_kw['apple'] += 1
count_by_kw['peach'] += 1
count_by_kw['orange'] += 1
count_by_kw['orange'] += 1

#Value in descending order (apple-> orange ->Take out the key to peach)
for kw, count in count_by_kw.most_common():
    print(kw, count)

Furthermore, by combining Counter and defaultdict, it is possible to create a dictionary of dictionaries. Of course, you can also use the most_common method.

`sample3.py`


# -*- coding: UTF-8 -*-
from collections import Counter
from collections import defaultdict

#Dictionary initialization
count_by_kw = defaultdict(Counter)

#Dictionary dictionary
count_by_kw['category1']['apple'] += 1
count_by_kw['category1']['apple'] += 1
count_by_kw['category1']['orange'] += 1

#Extract key and value from the outer dictionary, and then extract key and value from the inner dictionary
for kw1, counter in count_by_kw.items():
    for kw2, count in counter.most_common():
        print(kw1, kw2, count)