dict is a really nice data structure in python but it has limitations. I did some research recently with the collections module in Python and found two niche extentions of dict: OrderedDict and DefaultDict.
OrderedDict
OrderedDict serves under the circumstances that we want to keep the order of keys being added to the dictionary.
from collections import OrderedDict
items = (('apple', 2), ('pear', 1), ('orange', 3))
od = OrderedDict(items)
d = dict(items)
When we iterate over the dictionary, OrderedDict keeps the order while the normal dictionary doesn’t.
od.items()
Out[]: odict_items([('apple', 2), ('pear', 1), ('orange', 3)])
d.items()
Out[]: dict_items([('orange', 3), ('pear', 1), ('apple', 2)])
Or we can make a sorted dictionary with OrderedDict:
# sorted by key
sd = OrderedDict(sorted(items, key=lambda t: t[0]))
sd.items()
Out[]: odict_items([('apple', 2), ('orange', 3), ('pear', 1)])
DefaultDict
A non-trivial problem with dictionary in python is that when we query an item with d[key], it returns an error if the key doesn’t exist. In some implementations with dictionary, we don’t want this to happen - it’s ok that the key doesn’t exist.
With DefaultDict, we can assign a default factory method so we get default value when querying a non-existant key.
# make a defaultdict
from collections import defaultdict
items = (('apple', 2), ('pear', 1), ('orange', 3))
# set default factory method
dd = defaultdict(int)
for k, v in items:
dd[k] = v
# query a non-existant key
dd['mango']
Out[]: 0
There’s a lot of works that DefaultDict could do.
Store key-value pairs in lists:
items = [('GER', 'Bayern'), ('ENG', 'Man City'), ('GER', 'BVB'), ('ESP', 'Barcelona'), ('ENG', 'Liverpool')]
d = defaultdict(list)
for k, v in items:
d[k].append(v)
d.items()
Out[]: dict_items([('GER', ['Bayern', 'BVB']), ('ESP', ['Barcelona']), ('ENG', ['Man City', 'Liverpool'])])
Counting:
item = 'abgegangen'
d = defaultdict(int)
for k in item:
d[k] += 1
d.items()
Out[]: dict_items([('a', 2), ('e', 2), ('b', 1), ('n', 2), ('g', 3)])
More with collections module …
There are a few useful tools in the collections module such as Counter and deque. Find more in the official documentation.