stolid 2026.1.24.80598

Linter enforcing the conventions of the Python standard template

A flake8 plugin that enforces opinionated Python coding conventions for clean, maintainable code.

Stolid encourages:

• Composition over inheritance

• Dependency injection over mocking

• Protocol-based typing over abstract base classes

• Immutable dataclasses

• Small, focused functions and classes

Installation

pip install stolid

Usage

Stolid integrates directly with flake8:

flake8 your_code.py

Error Codes

SLD1xx - Testing

SLD102: Prohibits use of patch and patch.object from unittest.mock.

Use dependency injection instead of mocking:

# Bad
from unittest.mock import patch

@patch("mymodule.requests.get")
def test_fetch(mock_get):
    ...

# Good
class TestFetch(unittest.TestCase):
    def test_fetch(self) -> None:
        fake_client = FakeHTTPClient(response={"data": 1})
        fetcher = DataFetcher(client=fake_client)
        result = fetcher.fetch()
        assert_that(result, equal_to(1))

SLD2xx - Abstract Base Classes

SLD201: Prohibits importing ABC from the abc module.

SLD202: Prohibits using @abstractmethod decorator.

Use typing.Protocol for interfaces instead:

# Bad
from abc import ABC, abstractmethod

class HTTPClient(ABC):
    @abstractmethod
    def get(self, url: str) -> Response:
        ...

# Good
from typing import Protocol

class HTTPClient(Protocol):
    def get(self, url: str) -> Response:
        ...

SLD3xx - Object-Oriented Design

SLD301: Prohibits __init__ methods.

Use @dataclass with default_factory for attributes, or @classmethod for parameter computation:

# Bad
class Processor:
    def __init__(self, client, config):
        self.client = client
        self.config = config

# Good
from dataclasses import dataclass

@dataclass(frozen=True, slots=True, kw_only=True)
class Processor:
    client: HTTPClient
    config: Config

SLD302: Prohibits private methods (starting with _).

Extract private methods into separate classes:

# Bad
class Processor:
    def _validate(self, data):
        ...

# Good
class Validator:
    def validate(self, data):
        ...

SLD303: Flags methods that only access public members of self.

Convert to module-level functions or use functools.singledispatch:

# Bad
class User:
    name: str

    def display_name(self) -> str:
        return self.name.title()

# Good
def display_name(user: User) -> str:
    return user.name.title()

SLD4xx - Inheritance

SLD401: Prohibits inheritance from concrete classes.

Allowed base classes:

• Typing: Protocol, Generic

• Exceptions: Exception, BaseException

• Testing: TestCase

• Enums: Enum, IntEnum, StrEnum, Flag, IntFlag

• Other: TypedDict, NamedTuple

# Bad
class MyHandler(BaseHandler):
    ...

# Good
@dataclass(frozen=True, slots=True, kw_only=True)
class MyHandler:
    validator: Validator
    processor: Processor

SLD5xx - Dataclass Configuration

All dataclasses must have:

SLD501: frozen=True (immutability)

SLD502: slots=True (memory efficiency)

SLD503: kw_only=True (keyword-only arguments)

# Bad
@dataclass
class Settings:
    timeout: int

# Good
@dataclass(frozen=True, slots=True, kw_only=True)
class Settings:
    timeout: int

SLD6xx - Code Complexity

SLD601: Functions limited to 30 lines

SLD602: Functions limited to 4 arguments (excludes self/cls)

SLD603: Classes limited to 15 methods (excludes dunder methods)

SLD604: Modules limited to 400 lines

Configuration

Stolid follows standard flake8 configuration. Add to your setup.cfg or .flake8:

[flake8]
extend-ignore = SLD301,SLD302

Or use per-file ignores:

[flake8]
per-file-ignores =
    tests/*:SLD301,SLD302

Development

pip install nox

# Run all checks
nox

# Run specific sessions
nox -s tests    # Run tests with coverage
nox -s lint     # Run black and flake8
nox -s mypy     # Run type checking

License

MIT License. See LICENSE for details.