ALL-OVER0005 · abstract_member_left_unimplemented

Count promises a base made that the concrete subclass below it never kept.

This is a deterministic rule for all languages. Read its implementation.

Definition

Read every member a base declares that the subclass never declares again, and report one the base marked as abstract when the subclass is meant to be instantiated. An abstract member is a contract written down, and a concrete class that leaves one open has published a type that raises the moment anybody uses the part nobody finished. The failure lands on whoever instantiates the class rather than on whoever wrote it, which is the wrong person and usually the wrong week.

A class is read as abstract itself, and left alone, when it names a base such as ABC or Protocol anywhere above it or when it declares an abstract member of its own. Those are the two ways a class says it is a step on the way rather than a destination.

Evidence

Each finding names the subclass, the base that declared the promise, and the member left open. The value is the number of unkept promises.

Exceptions

A class naming one of abstract_bases anywhere in its inheritance chain is left alone, and a project whose own base spells that differently states its own set. A subclass that redeclares the name as data has answered the promise, since something is there to find.

Pylint also treats a body that only raises NotImplementedError as abstract, which is a convention rather than a declaration and leaves no decorator for a graph to read, so MCMR reports a subset of what Pylint reports rather than guessing at bodies. A metaclass passed as a keyword makes a class abstract too and leaves no inheritance edge behind, so that spelling is not seen either.

Examples

Bad

class Encoder:
    @abstractmethod
    def encode(self, value): ...


class JsonEncoder(Encoder):
    def describe(self):
        return "json"

Good

class JsonEncoder(Encoder):
    def encode(self, value):
        return json.dumps(value)

References

• Generalizes Pylint W0223 abstract-method. Open reference
• Cites “PEP 3119, Introducing Abstract Base Classes”. Open reference
• Cites “Design Patterns”, the template method pattern