ALL-OVER0004 · overriding_method_changes_its_call_protocol

Count overrides that change how a caller has to reach the member, not just what it does.

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

Definition

Read every member a base declares beside the declaration the subclass writes for it, and report an override that changed the protocol rather than the behavior. Two changes count. Turning a property into a method, or a method into a property, moves the parentheses to the call site, so thing.size starts returning a bound method that is always truthy and never the number anyone wanted. Turning an awaitable into a plain call, or the reverse, is worse still, because a coroutine nobody awaits is silently discarded and the work never happens.

Neither failure raises anything where it is written. Both surface far away, as a wrong number or as work that quietly did not run, and that distance is what makes them expensive.

Evidence

Each finding names the subclass, the base, the member, and how each side declared it. The value is the number of overrides that changed the protocol.

Exceptions

A name Python rewrites into the class that wrote it, spelled with two leading underscores and no trailing ones, is left alone, because no subclass can override it in the first place. A member one side writes as data is judged by the hiding rule rather than here.

A property implemented through a descriptor of the project’s own making is not recognized, since the graph reads the decorator a class wrote and not what that decorator returns. Pylint reports the property and the async halves as separate messages on one method, so a member that changed both counts once here and twice there.

Examples

Bad

class Source:
    async def read(self):
        return await self.stream.read()


class CachedSource(Source):
    def read(self):
        return self.buffer

Good

class CachedSource(Source):
    async def read(self):
        return self.buffer

References

• Generalizes Pylint W0236 invalid-overridden-method. Open reference
• Cites “PEP 492, Coroutines with async and await”. Open reference
• Cites “The Python Language Reference”, the descriptor protocol