Demonstrating Python's defaultdict

defaultdict_demo.py

"""
# Using Python's `collections.defaultdict`.

```python
>>> import collections
>>> dct = collections.defaultdict(int)
>>> int()
0
>>> dct['a']
0
>>> dct['b'] += 2
>>> dct
defaultdict(<class 'int'>, {'a': 0, 'b': 2})
>>> # Using custom defaults.
>>> dct2 = collections.defaultdict(lambda: [{}])
>>> dct2['a']
[{}]
>>> dct2['a'].append({'foo': 'bar'})
>>> dct2
defaultdict(<function <lambda> at 0x7f977bf96a60>, {'a': [{}, {'foo': 'bar'}]})
>>> # Creating defaultdict with a vanilla Python's dictionary.
>>> collections.defaultdict(int, {'a': 2, 'b': 3})
defaultdict(<class 'int'>, {'a': 2, 'b': 3})
>>>
```
"""


# Demonstrating Python's default dict.
#
# Problem:
#  Produce a mapping from {country: [sorted list of cities]}

import collections
import pprint

# Unsorted cities and countries.
cities_by_coutry = {
  'San Mateo': 'US',
  'Toronto': 'CA',
  'Detroit': 'US',
  'London': 'UK',
  'Paris': 'FR',
  'Seattle': 'US',
  'Vancouver': 'CA',
}


################################################################################################
# +--------------------------------------------------------------------------------------------+
# | Method one: Using vanilla Python dictionary.
# +--------------------------------------------------------------------------------------------+
################################################################################################
dct = {}
for city, country in cities_by_country.items():
  if country in dct:
    dct[country].append(city)  # append new city.
  else:
    dct[country] = [city]  # create a new list of cities.

# Sort list of cities.
for cities_list in dct.values():
  cities_list.sort()

# Pretty print the output.
pprint.pprint(dct)

################################################################################################
# +--------------------------------------------------------------------------------------------+
# | Method two: Using `collections.defaultdict`.
# +--------------------------------------------------------------------------------------------+
################################################################################################
dct = collections.defaultdict(list)
for city, country in cities_by_country.items():
  dct[country].append(city)

# Sort list of cities.
for cities in dct.values():
  cities.sort()

pprint.pprint(dct)

################################################################################################
# +--------------------------------------------------------------------------------------------+
# | Method three: Using `dict.setdefault` method.
# +--------------------------------------------------------------------------------------------+
################################################################################################
dct = {}
for city, country in cities_by_country.items():
  dct.setdefault(country, []).append(city)

for cities in dct.values():
  cities.sort()

pprint(dct)

thanks for writing this