factory_boy: testing like a pro. DjangoCon US 2022

@cmaiacd factory_boy DjangoCon US - Oct 2022 Camila Maia testing
like a pro

@cmaiacd You can ﬁnd this presentation at: speakerdeck.com/cmaiacd

@cmaiacd Who am I?

@cmaiacd Brazilian living in Berlin Bachelor of Computer Information System
Coding since 2010 👵 Python 🐍 and Ruby 💎 Open Source and Community ❤ Conferences Backend Developer @ SoundCloud

@cmaiacd Creator of ScanAPI

@cmaiacd First GitHub proﬁle to be accepted for the GitHub
Sponsors program in Brazil 󰎙 Quem sou eu?

@cmaiacd factory_boy: what is it?

@cmaiacd • It is a ﬁxtures replacement • Based on
factory_bot (Thoughtbot) • First version: Django only • Nowadays: framework-independent • Unittest, Pytest...

@cmaiacd For complex objects: ❌ Fixtures: static, hard to maintain
✅ Factories: easy-to-use

@cmaiacd ❌ Fixtures: exhaustive test setup with every possible combination
of corner cases ✅ Factories: objects customized for the current test, while only declaring the test-speciﬁc ﬁelds

@cmaiacd It offers many tools that help with the creation
of the tests: • Sequence • Faker • Fuzzy attributes • LazyFunction • LazyAttribute • Inheritance • Params • Traits • Strategies • RelatedFactory/SubFactory

@cmaiacd Demo App

@cmaiacd Polls

@cmaiacd Poll

@cmaiacd Results

@cmaiacd Model - Poll Poll Question Choice n:1 pub_date :
DateTimeField premium : BooleanField author : CharField question_text : CharField language : CharField choice_text : CharField votes : IntegerField 1:1

@cmaiacd Create Question - Admin

@cmaiacd Questions - Admin

@cmaiacd Model - Poll

@cmaiacd Model - Question

@cmaiacd Model - Choice

@cmaiacd Best Practices

@cmaiacd • +3 years • Django Monolith • +230 tables
• +2200 relevant ﬁles • +75k relevant lines My experience

@cmaiacd Bad factories are like viruses • Factories can get
too tied • Implicit errors might happen • Factory usage involves a lot of copy/paste with small changes • A poorly designed factory might affect many tests • The tests are created in a way to fit the factory (factory-oriented testing) • Developers bump into the same issue again and again • The same hack has to be done several times

@cmaiacd Bad factories are like viruses

@cmaiacd 1. FACTORIES SHOULD REPRESENT THEIR MODELS

@cmaiacd BAD ❌

@cmaiacd GOOD ✅

@cmaiacd 2. DO NOT RELY ON DEFAULTS FROM FACTORIES

@cmaiacd • If a default value is changed, all tests
that depend on it will break • The setup of a test should contain all the logic to ensure it will always pass • Explicit better than implicit

@cmaiacd BAD ❌

@cmaiacd GOOD ✅

@cmaiacd 3. FACTORIES SHOULD CONTAIN ONLY THE REQUIRED DATA

@cmaiacd If the ﬁeld is nullable (null=True) the attribute should
be under a trait and not as a default value

@cmaiacd BAD ❌

@cmaiacd GOOD ✅

@cmaiacd • If we want to have an author, we
can use PollFactory(with_author=True) now • When are we going to remember to test the case PollFactory(author=None)? • We should not assume there is an author when DB actually allows to not have it.

@cmaiacd 4. BUILD OVER CREATE

@cmaiacd • MyFactory.build() creates a local object (memory) • MyFactory.create()
creates a local object + stores it in the DB

@cmaiacd BAD ❌

@cmaiacd GOOD ✅

@cmaiacd BUILD STRATEGY ======================== 14 passed in 1.76 seconds =========================
CREATE STRATEGY ======================== 14 passed in 3.26 seconds =========================

@cmaiacd 5. IF FK IS IN THE TABLE: SUBFACTOR IF
FK IS IN THE OTHER TABLE: RELATEDFACTORY + TRAIT

@cmaiacd • SubFactory: builds/creates the SubFactory during the process of
creation of the main factory • RelatedFactory: builds/creates the RelatedFactory after creating the main factory

@cmaiacd Good ✅

@cmaiacd 6. USE FIXTURES TO WRAP FACTORIES TO AVOID DUPLICATION

@cmaiacd BAD ❌

@cmaiacd GOOD ✅

@cmaiacd 7. AVOID SHARING FACTORIES OR FIXTURES AMONG DIFFERENT FILES

@cmaiacd • Many tests depending on the same factory/fixture •
Tends to inflate the factory/fixture • Hard to maintain • Change a factory/fixture => tons of tests breaking • Fixture/Factory-oriented testing

@cmaiacd Código Code

@cmaiacd 1. Factories should represent their models 2. Do not
rely on defaults from factories 3. Factories should contain only the required data 4. Build over create 5. If FK is in the table: SubFactor If FK is in the other table: RelatedFactory + trait 6. Avoid sharing factories or fixtures among different files 7. Use fixtures to wrap factories to avoid duplication Review

@cmaiacd Documentation https://factoryboy.readthedocs.io/en/stable/ Common recipes https://factoryboy.readthedocs.io/en/stable/recipes.html Code https://github.com/FactoryBoy/factory_boy Links

@cmaiacd THANK YOU! @cmaiacd camilamaia 󰠁 cmaiacd.com

factory_boy: testing like a pro. DjangoCon US 2022

factory_boy: testing like a pro. DjangoCon US 2022

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