Flake8 ElegantObjects Plugin

Detects violations of Elegant Objects principles including the "-er" naming principle, null usage, mutable objects, code in constructors, and getter/setter patterns.
Error Codes
Naming Violations (EO001-EO004)
EO001
: Class name violates -er principle
EO002
: Method name violates -er principle
EO003
: Variable name violates -er principle
EO004
: Function name violates -er principle
Object Behavior (EO005-EO007)
EO005
: Null (None) usage violates EO principle
EO006
: Code in constructor violates EO principle
EO007
: Getter/setter method violates EO principle
Mutable Object Violations (EO008, EO015-EO027)
EO008
: Mutable dataclass violation
EO015
: Mutable class attribute violation
EO016
: Mutable instance attribute violation
EO017
: Instance attribute mutation violation
EO018
: Augmented assignment mutation violation
EO019
: Mutating method call violation
EO020
: Subscript assignment mutation violation
EO021
: Chained mutation violation
EO022
: Missing factory methods violation
EO023
: Mutable default argument violation
EO024
: Missing immutability enforcement violation
EO025
: Copy-on-write violation
EO026
: Aliasing violation (exposing mutable state)
EO027
: Defensive copy violation
Design and Architecture (EO009-EO014)
EO009
: Static method violates EO principle (no static methods allowed)
EO010
: isinstance/type casting violates EO principle (avoid type discrimination)
EO011
: Public method without contract (Protocol/ABC) violates EO principle
EO012
: Test method contains non-assertThat statements (only assertThat allowed)
EO013
: ORM/ActiveRecord pattern violates EO principle
EO014
: Implementation inheritance violates EO principle
Installation
pip install flake8-elegant-objects
Usage
Standalone:
python -m flake8_elegant_objects path/to/files/*.py
python -m flake8_elegant_objects --show-source path/to/files/*.py
As flake8 plugin:
flake8 --select=EO path/to/files/
The plugin is automatically registered when the package is installed.
Philosophy
Based on Yegor Bugayenko's Elegant Objects principles, this plugin enforces object-oriented design that treats objects as living, thinking entities rather than data containers or procedure executors.
1. No "-er" Entities (EO001-EO004)
Why? Names ending in "-er" describe what objects do rather than what they are, reducing them to mechanical task performers instead of equal partners in your design.
- ❌
class DataProcessor
→ ✅ class ProcessedData
- ❌
def analyze()
→ ✅ def analysis()
- ❌
parser = ArgumentParser()
→ ✅ arguments = ArgumentParser()
2. No Null/None (EO005)
Why? Null references break object-oriented thinking by representing "absence of object" - but absence cannot participate in object interactions. They lead to defensive programming and unclear contracts.
- ❌
return None
→ ✅ Return null objects with safe default behavior
- ❌
if user is None:
→ ✅ Use null object pattern or throw exceptions
3. No Code in Constructors (EO006)
Why? Constructors should be about object assembly, not computation. Complex logic in constructors violates the principle that objects should be lazy and do work only when asked.
- ❌
self.name = name.upper()
→ ✅ self.name = name
(transform on access)
- ❌
self.items = [process(x) for x in data]
→ ✅ self.data = data
(process lazily)
Why? Getters and setters expose internal state, breaking encapsulation. They encourage "tell, don't ask" violations and treat objects as data containers rather than behavioral entities.
- ❌
def get_value()
/ def set_value()
→ ✅ Objects should expose behavior, not data
- ❌
user.getName()
→ ✅ user.introduce_yourself()
or user.greet(visitor)
5. No Mutable Objects (EO008, EO015-EO027)
Why? Mutable objects introduce temporal coupling and make reasoning about code difficult. Immutable objects are thread-safe, predictable, and easier to test. This plugin provides comprehensive detection of various mutability patterns.
Basic Mutability Issues:
- ❌
@dataclass class Data
→ ✅ @dataclass(frozen=True) class Data
(EO008)
- ❌
items = []
(class attribute) → ✅ items: tuple = ()
(EO015)
- ❌
self.data = []
(instance attribute) → ✅ self.data: tuple = ()
(EO016)
Mutation Patterns:
- ❌
self.items.append(x)
→ ✅ self.items = (*self.items, x)
(EO019)
- ❌
self.count += 1
→ ✅ return Counter(self.count + 1)
(EO018)
- ❌
self.data[key] = value
→ ✅ Use immutable data structures (EO020)
- ❌
self.data = new_value
(after init) → ✅ Return new instance (EO017)
Advanced Patterns:
- ❌
def items=[]:
(mutable defaults) → ✅ def items=None:
+ null object (EO023)
- ❌
return self._items
(exposing mutable state) → ✅ return tuple(self._items)
(EO026)
- ❌
self.items = items
(no defensive copy) → ✅ self.items = tuple(items)
(EO027)
- ❌ Class with mutable state but no factory methods → ✅ Provide immutable factory methods (EO022)
6. No Static Methods (EO009)
Why? Static methods belong to classes, not objects, breaking object-oriented design. They can't be overridden, can't be mocked easily, and promote procedural thinking. Every static method is a candidate for a new class.
- ❌
@staticmethod def process()
→ ✅ Create dedicated objects for behavior
- ❌
Math.sqrt(x)
→ ✅ SquareRoot(x).value()
7. No Type Discrimination (EO010)
Why? Using isinstance
, type casting, or reflection is a form of object discrimination. It violates polymorphism by treating objects unequally based on their type rather than their behavior contracts.
- ❌
isinstance(obj, str)
→ ✅ Design common interfaces and use polymorphism
- ❌
if type(x) == int:
→ ✅ Let objects decide how to behave
8. No Public Methods Without Contracts (EO011)
Why? Public methods without explicit contracts (Protocol/ABC) create implicit dependencies and unclear expectations. Contracts make object collaboration explicit and testable.
- ❌
class Service:
with ad-hoc public methods → ✅ class Service(Protocol):
with defined contracts
- ❌ Implicit interfaces → ✅ Explicit protocols that can be tested and verified
9. Test Methods: Only assertThat (EO012)
Why? Test methods should contain only one assertion statement (preferably assertThat
). Multiple statements create complex tests that are hard to understand and maintain. Each test should verify one specific behavior.
- ❌
x = 5; y = calculate(x); assert y > 0
→ ✅ assertThat(calculate(5), is_(greater_than(0)))
- ❌ Multiple assertions per test → ✅ One focused assertion per test
10. No ORM/ActiveRecord (EO013)
Why? ORM and ActiveRecord patterns mix data persistence concerns with business logic, violating single responsibility. They create anemic domain models and tight coupling to databases.
- ❌
user.save()
, Model.find()
→ ✅ Separate repository objects
- ❌ Mixing persistence with business logic → ✅ Clean separation of concerns
11. No Implementation Inheritance (EO014)
Why? Implementation inheritance creates tight coupling between parent and child classes, making code fragile and hard to test. It violates composition over inheritance and creates deep hierarchies that are difficult to understand.
- ❌
class UserList(list):
→ ✅ class UserList:
with composition
- ❌ Inheriting concrete implementations → ✅ Inherit only from abstractions (ABC/Protocol)
The plugin detects the "hall of shame" naming patterns: Manager, Controller, Helper, Handler, Writer, Reader, Converter, Validator, Router, Dispatcher, Observer, Listener, Sorter, Encoder, Decoder, Analyzer, etc.
Configuration
The plugin is integrated with flake8. Add to your .flake8
config:
[flake8]
select = E,W,F,EO
per-file-ignores =
tests/*:EO012
Development
Testing
Run all tests:
python -m pytest tests/ -v
Code Quality
mypy flake8_elegant_objects/
ruff check flake8_elegant_objects/
ruff format flake8_elegant_objects/
Project Structure
flake8_elegant_objects/
├── __init__.py # Main plugin entry point
├── __main__.py # CLI interface
├── base.py # Core types, error codes, and base classes
├── no_constructor_code.py # EO006: No code in constructors
├── no_er_name.py # EO001-EO004: No -er naming violations
├── no_getters_setters.py # EO007: No getter/setter methods
├── no_implementation_inheritance.py # EO014: No implementation inheritance
├── no_impure_tests.py # EO012: Test methods with single assertions
├── no_null.py # EO005: No None/null usage
├── no_orm.py # EO013: No ORM/ActiveRecord patterns
├── no_public_methods_without_contracts.py # EO011: Methods need contracts
├── no_static.py # EO009: No static methods
├── no_type_discrimination.py # EO010: No isinstance/type casting
└── no_mutable_objects/ # EO008, EO015-EO027: Comprehensive mutability detection
├── __init__.py # Package initialization
├── base.py # Shared utilities and state tracking
├── core.py # Main orchestrator for all mutable object checks
├── contract_checker.py # EO024: Immutability contract enforcement
├── copy_on_write_checker.py # EO025: Copy-on-write pattern validation
├── deep_checker.py # Cross-class mutation analysis
├── factory_checker.py # EO022: Factory method pattern validation
├── pattern_detectors.py # EO026-EO027: Aliasing and defensive copy detection
└── shared_state_checker.py # EO023: Shared mutable state detection