Using Python collections defaultdict, OrderedDict, and Sorting Techniques

This guide introduces advanced usage of the Python collections module, focusing on defaultdict , OrderedDict , and sorting methods for dictionaries and dictionary lists.

1. defaultdict for mapping multiple values

Given a list of key‑value pairs, a normal dict can store only one value per key. By using defaultdict(list) (or defaultdict(set) ), each key can accumulate multiple values automatically.

<code>l = [('a',2),('b',3),('a',1),('b',4),('a',3),('a',1),('b',3)]
from collections import defaultdict
d = defaultdict(list)
for key, value in l:
    d[key].append(value)
# Result: defaultdict(list, {'a': [2, 1, 3, 1], 'b': [3, 4, 3]})
</code>

If you need unique values, replace list with set . The same pattern works for counting occurrences by using defaultdict(int) :

<code>from collections import defaultdict
objs = [{'F29958SVDK6': 12}, {'F29958SVDK6': 12}, {'F29958SVDK6': 12}, {'F29958SVDK6': 12}, {'F29958SVDK6': 12}]
d = defaultdict(int)
for obj in objs:
    for key, value in obj.items():
        d[key] += value
# Result: defaultdict(int, {'F29958SVDK6': 60})
</code>

2. OrderedDict for preserving insertion order

Although regular dicts are ordered since Python 3.7, the classic OrderedDict still provides explicit methods such as move_to_end and popitem .

<code>from collections import OrderedDict
d = OrderedDict()
d['bar'] = 2
d['non'] = 8
d['sek'] = 5
# Move 'bar' to the end
d.move_to_end('bar')
# d is now OrderedDict([('non', 8), ('sek', 5), ('bar', 2)])
# Remove the first item
first_item = d.popitem(last=False)  # ('non', 8)
</code>

3. Sorting dictionaries

The built‑in sorted function can order dictionary items by key or by value.

<code>d = {'b': 3, 'a': 4, 'c': 2, 'd': 1}
# Sort by key
sorted_by_key = sorted(d.items(), key=lambda i: i[0])
# [('a', 4), ('b', 3), ('c', 2), ('d', 1)]
# Sort by value
sorted_by_value = sorted(d.items(), key=lambda i: i[1])
# [('d', 1), ('c', 2), ('b', 3), ('a', 4)]
</code>

4. Sorting a list of dictionaries

When you have a list of dicts, operator.itemgetter makes it easy to sort by one or multiple fields.

<code>from operator import itemgetter
rows = [
    {'fname': 'Brian', 'lname': 'Jones', 'uid': 1003},
    {'fname': 'David', 'lname': 'Beazley', 'uid': 1002},
    {'fname': 'John',  'lname': 'Cleese',  'uid': 1001},
    {'fname': 'Big',   'lname': 'Jones',   'uid': 1004}
]
# Sort by first name
rows_by_fname = sorted(rows, key=itemgetter('fname'))
# Sort by uid
rows_by_uid = sorted(rows, key=itemgetter('uid'))
# Sort by last name then first name
rows_by_lfname = sorted(rows, key=itemgetter('lname', 'fname'))
</code>

These techniques cover common advanced dictionary operations in Python, helping you group data, maintain order, and perform flexible sorting.