Group items by key in Python

SQL's GROUP BY, in three lines of Python.

from collections import defaultdict

def group_by(items, key):
    groups = defaultdict(list)
    for item in items:
        groups[key(item)].append(item)
    return dict(groups)

How it works

1. defaultdict(list) creates a bucket the first time a key appears.
2. Each item lands in the bucket its key function picks.
3. Returns the groups as a plain dict.

Keywords and builtins used here

• def
• dict
• for
• group_by
• list
• return

The run, in numbers

Lines

7

Characters to type

159

Tokens

40

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 24 seconds.

Type this snippet

Step 10 of 10 in Collections, step 32 of 72 in Language basics.

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