pylint-enums 0.0.5

A Pylint plugin that checks for a specific implementation of Enum subclasses.

Pylint-Enums

Installation:

$ pip install pylint_enums     # a conventional option
$ pipenv install pylint_enums  # a more modern option

Usage:

• Follow instructions to add pylint_enums as part of your loaded plugins.
  • Option 1 (.pylintrc)
    • Add load-plugins=pylint_enums to your .pylintrc.
    • Use pylint normally, i.e., $ pylint [filepath]
  • Option 2 (command line option)
    • $ pylint [filepath] --load-plugins=pylint_enums

What this is:

This is a tiny pylint plugin that adds a checker for Enum subclasses. It warns you when you haven't provided a typed annotation for the value attribute and when a __str__ method has not been defined for the Enum when the type annotation isn't part of a finite list of simple types.

__str__ declaration is not enforced for the following types:

• str
• int
• decimal
• float

Why this is helpful:

Typically, the value of an enum doesn't matter.

from enum import Enum

class Foo(Enum):
    FIRST = 'these'
    SECOND = 'usually'
    THIRD = 'don\'t'
    FOURTH = 'matter'

However, in certain applications and/or use-cases, you actually do care about the value of each enum member. Specifically, you may use them as a verbose_name or a pretty_name for displaying to the user, or you may want to assign it a stateful value and use its contents later.

from enum import Enum
from typing import NamedTuple

class FooMember(NamedTuple):
    label: str
    rank: int

class Foo(Enum):
    FIRST = FooMember(label='first', rank=1)
    SECOND = FooMember(label='second', rank=2)
    THIRD = FooMember(label='third', rank=3)
    FOURTH = FooMember(label='fourth', rank=4)

    def __str__(self) -> str:
        return self.value.label

As of this writing, mypy==0.600 is unable to infer the types of the member values. They resolve to 'Any':

reveal_type(Foo)             # 'def (value: Any) -> foo.Foo'
reveal_type(Foo.FIRST)       # 'foo.Foo'
reveal_type(Foo.FIRST.value) # 'Any'

This can be problematic for mypy users that rely on type hints to maintain their code base. When you write a function that returns Foo.FIRST.value, our tooling is unable to help us figure out whether this value is a str, a NamedTuple, or some other value. Ideally, we would add additional type hints to the Enum:

class Foo(Enum):
    value: FooMember
    FIRST = FooMember(label='first', rank=1)
    ...

But alas, this requires developer vigilance to remember to do. If you're maintaining many enums across multiple files, it could be annoying to make sure that they and all future defined Enums are adequately typed.

This pylint plugin will raise errors when value is not typed and when the value is typed to something complex and the Enum is missing a __str__ method.

Author

Christopher Sabater Cordero