Python ABC: prevent construction of an incomplete implementation

Operation contract

The writer interface requires a publish method. An incomplete subclass fails during construction; the local writer implements the operation and returns a receipt identifier. The abstract requirement checks that an implementation exists, not that every accepted input or returned value satisfies the caller’s domain contract.

Failure and ownership boundary

Abstract registration and structural Protocol annotations serve different purposes. A concrete override can still have an incompatible signature or wrong behavior at runtime. Test the collaborator’s accepted values and failures separately; Python Protocol: structural intent without runtime validation and Python unittest.mock: inject a collaborator failure before publication address those obligations.

Working program

from abc import ABC, abstractmethod

class ReceiptWriter(ABC):
    @abstractmethod
    def publish(self, receipt):
        raise NotImplementedError

class MissingWriter(ReceiptWriter):
    pass

class LocalReceiptWriter(ReceiptWriter):
    def publish(self, receipt):
        return receipt["receipt_id"]

try:
    MissingWriter()
except TypeError:
    print("incomplete writer rejected")
print(LocalReceiptWriter().publish({"receipt_id": "R-0041"}))

Output

incomplete writer rejected
R-0041

Costs and limits

Abstract method bookkeeping happens during class construction and instantiation checks. Actual method cost is implementation-dependent. The fixture performs no storage publication despite its interface name; its writer returns a local value only.

Common Mistakes

• A concrete override is not proof of behavioral compatibility.
• Do not confuse an ABC with payload validation.

Connected lessons

Python Protocol: structural intent without runtime validation, Python unittest.mock: inject a collaborator failure before publication, Python metaclasses: validate a class before registering its wire format.