Using Pythons's reduce function to group a list of dictionaries by a specific key

example.py

#!/usr/bin/env python3.7
#
# Copyright (C) 2020 Erlend Ekern <[email protected]>
#
# Distributed under terms of the MIT license.

"""
An example of using `functools.reduce` to group a list of dictionaries by a specific key.
"""

from functools import reduce

purchases = [
    {"customer_id": "0", "product_id": "0"},
    {"customer_id": "0", "product_id": "1"},
    {"customer_id": "1", "product_id": "2"},
    {"customer_id": "2", "product_id": "2"},
    {"customer_id": "0", "product_id": "3"},
]
group_by_key = "customer_id"
group_purchases_by_key = reduce(
    lambda acc, curr: {
        **acc,
        curr[group_by_key]: acc.get(curr[group_by_key], []) + [curr],
    },
    purchases,
    {},
)
print(group_purchases_by_key)