Protocol Support¶

Purpose¶

The library SHALL provide protocol classes with immutability and concealment that work with structural subtyping, preserving protocol functionality including runtime checks.

Requirements¶

Requirement: Protocol Metaclass¶

The Protocol metaclass MUST combine typing.Protocol with standard behaviors.

Scenario: Protocol class creation¶

WHEN a class uses the Protocol metaclass
THEN the resulting class MUST support structural subtyping with immutability and concealment

Requirement: Protocol Base Class¶

ProtocolObject MUST serve as a base class for protocol definitions with standard behaviors.

Scenario: Protocol inheritance¶

WHEN a class inherits from ProtocolObject
THEN the resulting class MUST be a protocol with immutability and concealment

Requirement: Protocol Decorator¶

The protocol_with_standard_behaviors decorator MUST create protocol classes with standard behaviors.

Scenario: Decorated protocol¶

WHEN @protocol_with_standard_behaviors() is applied to a class
THEN the resulting class MUST be a protocol with immutability and concealment

Requirement: Structural Subtyping Preservation¶

Protocol functionality MUST be preserved, including structural subtyping and runtime checks.

Scenario: Structural subtyping¶

WHEN a class implements all protocol methods
THEN isinstance() MUST return True for that protocol

Scenario: Runtime checkable protocols¶

WHEN a protocol is decorated with @runtime_checkable
THEN isinstance() checks MUST work correctly

Requirement: ABC Attribute Exemptions¶

ABC-related mutable attributes (_abc_cache, _abc_registry, etc.) MUST be allowed by default to ensure protocol functionality.

Scenario: ABC cache mutation¶

WHEN Python internals modify _abc_cache on a protocol class
THEN no immutability error MUST be raised

Requirement: Implementation Inheritance¶

Immutability and concealment MUST be inherited by classes implementing protocol interfaces.

Scenario: Implementation inherits behaviors¶

WHEN a class implements a protocol with standard behaviors
THEN the implementation MUST inherit immutability and concealment