Here\'s a suitable title for your article, keeping the question format in mind: How to Group Data Pairs by Type in Python: Efficient Solutions with `defaultdict` and `itertools.groupby`

Python Group By

Problem:

Given a set of data pairs where the first item is the value and the second item is the type, group them based on the type.

Example:

input = [
  ('11013331', 'KAT'),
  ('9085267', 'NOT'),
  ('5238761', 'ETH'),
  ('5349618', 'ETH'),
  ('11788544', 'NOT'),
  ('962142', 'ETH'),
  ('7795297', 'ETH'),
  ('7341464', 'ETH'),
  ('9843236', 'KAT'),
  ('5594916', 'ETH'),
  ('1550003', 'ETH'),
]

Desired Result:

result = [
  {
    'type': 'KAT',
    'items': ['11013331', '9843236']
  },
  {
    'type': 'NOT',
    'items': ['9085267', '11788544']
  },
  {
    'type': 'ETH',
    'items': ['5238761', '962142', '7795297', '7341464', '5594916', '1550003']
  },
]

Solution:

Step 1: Create a Dictionary

• Use the collections.defaultdict to create a dictionary where keys are item types and values are lists to store values.
• For each data pair, append the value to the list corresponding to its type.

Step 2: Convert to Expected Format

• Convert the dictionary into the expected format by iterating over its items and creating dictionaries with 'type' and 'items' keys.

Example Code:

<code class="python">from collections import defaultdict

res = defaultdict(list)
for v, k in input:
    res[k].append(v)

output = [{'type': k, 'items': v} for k, v in res.items()]</code>

Alternative Solution using itertools.groupby:

Note: This approach requires the input to be sorted.

• Sort the data pairs based on their types.
• Use itertools.groupby to group data pairs by type.
• Iterate over the grouped items to create the desired output format.

Example Code:

<code class="python">from itertools import groupby, itemgetter

sorted_input = sorted(input, key=itemgetter(1))

groups = groupby(sorted_input, key=itemgetter(1))
output = [{'type': k, 'items': [x[0] for x in v]} for k, v in groups]</code>

Note on Key Order:

• In Python versions before 3.7, insertion order is not guaranteed for dictionaries. To keep the order consistent with the input, use collections.OrderedDict.
• In Python 3.7 and later, dictionaries preserve insertion order by default.

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