Blog Post Publish Date: 2025/07/28
groupby + attrgetter: The Pythonic Way to Group Objects#
This blog post explores an approach to group objects by mulitple attribute name powered by capabilities of the built-in modules itertools.groupby and operator.attrgetter module.
To illustrate this scenario, let’s create a Band class, and a bands variable with a list of the instances. The objective is create groups by attributes category, genre, country and print the name.
from dataclasses import dataclass
@dataclass
class Band:
name: str
category: str
genre: str
country: str
bands = [
Band(name="The Beatles", category="Rock", genre="Rock", country="UK"),
Band(name="Queen", category="Rock", genre="Rock", country="UK"),
Band(name="The Rolling Stones", category="Rock", genre="Rock", country="UK"),
Band(name="Led Zeppelin", category="Rock", genre="Rock", country="UK"),
Band(name="The Who", category="Rock", genre="Rock", country="UK"),
Band(name="Deep Purple", category="Rock", genre="Rock", country="UK"),
Band(name="Nirvana", category="Rock", genre="Grunge", country="USA"),
Band(name="Temple of the Dog", category="Rock", genre="Grunge", country="USA"),
Band(name="Pearl Jam", category="Rock", genre="Grunge", country="USA"),
Band(name="Soundgarden", category="Rock", genre="Grunge", country="USA"),
Band(name="Alice in Chains", category="Rock", genre="Grunge", country="USA"),
Band(name="Stone Temple Pilots", category="Rock", genre="Grunge", country="USA"),
Band(name="Pink Floyd", category="Rock", genre="Progressive Rock", country="UK"),
Band(name="Genesis", category="Rock", genre="Progressive Rock", country="UK"),
Band(name="Yes", category="Rock", genre="Progressive Rock", country="UK"),
Band(name="King Crimson", category="Rock", genre="Progressive Rock", country="UK"),
Band(name="The Smiths", category="Rock", genre="Indie Rock", country="UK"),
Band(name="Arctic Monkeys", category="Rock", genre="Indie Rock", country="UK"),
Band(name="Pet Shop Boys", category="Pop", genre="Synth-pop", country="UK"),
Band(name="Depeche Mode", category="Pop", genre="Synth-pop", country="UK"),
Band(name="Erasure", category="Pop", genre="Synth-pop", country="UK"),
Band(name="Korine", category="Pop", genre="Synth-pop", country="USA"),
Band(name="Oasis", category="Rock", genre="Britpop", country="UK"),
Band(name="Blur", category="Rock", genre="Britpop", country="UK"),
Band(name="Pulp", category="Rock", genre="Britpop", country="UK"),
Band(name="Echo & the Bunnymen", category="Rock", genre="Post-Punk", country="UK"),
Band(name="Joy Division", category="Rock", genre="Post-Punk", country="UK"),
Band(name="The Cure", category="Rock", genre="Post-Punk", country="UK"),
Band(name="The Killers", category="Rock", genre="Alternative Rock", country="USA"),
Band(name="R.E.M.", category="Rock", genre="Alternative Rock", country="USA"),
Band(name="Radiohead", category="Rock", genre="Alternative Rock", country="UK"),
Band(name="U2", category="Rock", genre="Rock", country="Ireland"),
]
The most common approach is to create a temporary dictionary where the key is a tuple of the attributes you want to group by, and the value is a list of objects that share those attributes:
bands_groupby_genre_country = {}
for band in bands:
group_key = (band.genre, band.country, band.category)
bands_groupby_genre_country.setdefault(group_key, []).append(band)
for (category, genre, country), band_group in bands_groupby_genre_country.items():
print(f">>> {category=}, {genre=}, {country=}:")
for band in band_group:
print(f"- {band.name}")
This approach works, but the code readability can cause confusion. Therefore, I present a more pythonic way to perform this operation using groupby and attrgetter.
The groupby function takes an iterable as its first positional argument and has an optional named argument key. The key works exactly the same way as in sorted, accepting a function that is called to determine the grouping. In the example below, the function is created using a lambda, where the groups are formed based on the category, genre, and country attributes.
from itertools import groupby
attribute_group = lambda item: (item.category, item.genre, item.country)
for (category, genre, country), group in groupby(bands, key=attribute_group):
print(f">>> {category=}, {genre=}, {country=}:")
for band in group:
print(f" - {band.name}")
This approach offers better readability. However, by using the attrgetter function, we can eliminate the lambda and make the code more self-explanatory. The attrgetter returns a callable object that fetches the specified attribute from its operand. If more than one attribute is requested, it returns a tuple of attributes. For practical purposes, this solution works the same way as the lambda, but the code is clearer and easier to understand.
from operator import attrgetter
from itertools import groupby
attribute_group = attrgetter("category", "genre", "country",)
for (category, genre, country), group in groupby(bands, key=attribute_group):
print(f">>> {category=}, {genre=}, {country=}:")
for band in group:
print(f" - {band.name}")
The script execution output will be:
>>> category='Rock', genre='Rock', country='UK':
- The Beatles
- Queen
- The Rolling Stones
- Led Zeppelin
- The Who
- Deep Purple
>>> category='Rock', genre='Grunge', country='USA':
- Nirvana
- Temple of the Dog
- Pearl Jam
- Soundgarden
- Alice in Chains
- Stone Temple Pilots
>>> category='Rock', genre='Progressive Rock', country='UK':
- Pink Floyd
- Genesis
- Yes
- King Crimson
>>> category='Rock', genre='Indie Rock', country='UK':
- The Smiths
- Arctic Monkeys
>>> category='Pop', genre='Synth-pop', country='UK':
- Pet Shop Boys
- Depeche Mode
- Erasure
>>> category='Pop', genre='Synth-pop', country='USA':
- Korine
>>> category='Rock', genre='Britpop', country='UK':
- Oasis
- Blur
- Pulp
>>> category='Rock', genre='Post-Punk', country='UK':
- Echo & the Bunnymen
- Joy Division
- The Cure
>>> category='Rock', genre='Alternative Rock', country='USA':
- The Killers
- R.E.M.
>>> category='Rock', genre='Alternative Rock', country='UK':
- Radiohead
>>> category='Rock', genre='Rock', country='Ireland':
- U2
- Thin Lizzy
For the full working example, visit the GitHub repository. You can also run the code directly in your terminal:
curl -s https://raw.githubusercontent.com/c-neto/my-code-playground/refs/heads/main/blog-code-examples/2025-07-28-groupby/python-groupby-example.py | python -