fport

Version

Overview

A generic unidirectional function coupling module based on loose coupling.

This module provides an interface for the implementation side to send information.

Main Purpose and Use Cases

• Submitting information from the implementation side for white-box testing
• Creating entry points for simple add-ons

Supported Environment

• Python 3.10 or later
• No external dependencies

License

This module is provided under the MIT License. See the LICENSE file for details.

Installation

PyPI

pip install fport

GitHub

pip install git+https://github.com/minoru_jp/fport.git

Features

• Provides a communication channel to the implementation side with minimal setup.
• Designed so that the sending interface has no side effects on the implementation side (only computation cost on the receiving side).
• The sending interface does not propagate errors from the receiver or framework to the implementation side.
• The sending interface always returns None.
• The scope of information transfer can be flexibly defined by where and how the interface is defined and shared.
• You can configure the sending interface to reject connections.
• Even if the connection is rejected, the implementation side always gets a valid interface.

Warning

The communication mechanism adopted by this module is loosely coupled and does not explicitly specify the destination from the implementation side. Information transmitted from the implementation must be carefully considered. Careless transmission may lead to leaks of authentication data, personal information, or other critical data. The same applies to information that can be reconstructed into such sensitive data.

About Parallel Processing

The sending interface is thread-unsafe. This design avoids unintended serialization on the implementation side. Maintaining overall consistency, including the use of interfaces in parallel processing, is the responsibility of the implementation side.

Simple Usage Example

from fport import create_session_policy

policy = create_session_policy()
port = policy.create_port()

def add(a, b):
    port.send("add", a, b)
    return a + b

def listener(tag, *args, **kwargs):
    print("Received:", tag, args, kwargs)

with policy.session(listener, port) as state:
    result = add(2, 3)
    print("Result:", result)

# Output:
# Received: add (2, 3) {}
# Result: 5

---

Main API Reference

create_session_policy(*, block_port: bool = False, message_validator: SendFunction | None = None) -> SessionPolicy

Factory function to generate a SessionPolicy.

• Parameters

  • block_port: bool If True, all Ports created by this policy reject connections.
  • message_validator: SendFunction | None Optional validation function for sending. Called before Port.send(). If an exception is raised, the send is rejected. The exception does not propagate to the sender; instead, it is treated as a session termination.

• Returns SessionPolicy

---

class SessionPolicy

Interface for managing Port creation and session establishment.

• Methods

  • create_port() -> Port Creates a connectable Port.

  • create_noop_port() -> Port Creates a no-op Port that rejects connections.

  • session(listener: ListenFunction, target: Port) -> ContextManager[SessionState] Returns a context manager to start a session by connecting listener to the specified Port.

    • Parameters

      • listener: ListenFunction A callback function that receives messages sent via Port.send(). Takes arguments (tag: str, *args, **kwargs).
      • target: Port The target Port instance.

    • Returns ContextManager[SessionState] Used in a with block. Provides SessionState for monitoring with ok and error.

    • Exceptions

      • TypeError: If target is not a Port instance
      • OccupiedError: If the specified Port is already used by another session
      • DeniedError: If the Port or SessionPolicy is set to reject connections
      • RuntimeError: Unexpected internal inconsistencies

---

class Port

Interface for the implementation (sending side) to transmit information.

• Methods

  • send(tag: str, *args, **kwargs) -> None Sends arbitrary information to registered listeners.

    • Does nothing if no listener is registered
    • Exceptions are not propagated to the sender (fail-silent)
    • Thread-unsafe: designed to avoid unintended serialization

---

class SessionState

Read-only interface for monitoring session status.

• Properties

  • ok: bool Whether the session is still active
  • error: Exception | None The first error that caused the session to end, or None

---

Exceptions

• class DeniedError(Exception) Raised when a policy or Port rejects a connection.

• class OccupiedError(Exception) Raised when a Port is already occupied by another session.

---

Protocols (Types)

• class SendFunction(Protocol)

  def __call__(tag: str, *args, **kwargs) -> None

  Callable object used by the sender to send messages.

• class ListenFunction(Protocol)

  def __call__(tag: str, *args, **kwargs) -> None

  Callable object used by the receiver to process messages.

---

Observer

This library includes an observer implementation as a listener.

Example usage with fport

from fport import create_session_policy
from fport.observer import ProcessObserver

def create_weather_sensor(port):
    """Weather sensor
    Specification:
        temp < 0        -> "Freezing" + send("freezing")
        0 <= temp <= 30 -> "Normal"   + send("normal")
        temp > 30       -> "Hot"      + send("hot")
    """
    def check_weather(temp: int) -> str:
        # If there is a bug here, it will be detected by the test
        if temp <= 0:   # ← Common place to inject a bug
            port.send("freezing", temp)
            return "Freezing"
        elif temp <= 30:
            port.send("normal", temp)
            return "Normal"
        else:
            port.send("hot", temp)
            return "Hot"
    return check_weather

policy = create_session_policy()
port = policy.create_port()

# Define expected conditions according to the specification
conditions = {
    "freezing": lambda t: t < 0,
    "normal":   lambda t: 0 <= t <= 30,
    "hot":      lambda t: t > 30,
}
observer = ProcessObserver(conditions)
check_weather = create_weather_sensor(port)

with policy.session(observer.listen, port) as state:
    # Test coverage for all three branches
    for i in (-5, 0, 31):
        check_weather(i)
        if not state.ok:
            raise AssertionError(f"observation failed on '{i}'")

    # Verify that the Observer did not detect any specification violations
    if observer.violation:
        details = []
        for tag, obs in observer.get_violated().items():
            details.append(
                f"[{tag}] reason={obs.fail_reason}, "
                f"count={obs.count}, first_violation_at={obs.first_violation_at}"
            )
        raise AssertionError("Observer detected violations:\n" + "\n".join(details))

print("All checks passed!")

---

Observer API Reference

Class ProcessObserver

Monitors process state, handling condition violations and exceptions.

Constructor

ProcessObserver(conditions: dict[str, Callable[..., bool]])

Initializes with the given set of conditions to monitor.

Methods

• reset_observations() -> None Reset all observation results.

• listen(tag: str, *args, **kwargs) -> None Evaluate the condition for the given tag. Calls handlers on violation or exception.

• get_all() -> dict[str, Observation] Returns all observation results.

• get_violated() -> dict[str, Observation] Returns observations where violations occurred.

• get_compliant() -> dict[str, Observation] Returns observations with no violations.

• get_unevaluated() -> dict[str, Observation] Returns unevaluated observations.

• set_violation_handler(tag: str, fn: Callable[[Observation], None]) -> None Sets a violation handler for the specified tag.

• set_exception_handler(fn: Callable[[str, ExceptionKind, Observation | None, Exception], None]) -> None Sets an exception handler.

• get_stat(tag: str) -> ConditionStat Returns statistical information for the specified tag.

Properties

• violation: bool Whether any violation exists.

• global_violation: bool Whether any global violation exists.

• local_violation: bool Whether any local violation exists.

• global_fail_reason: str Returns the reason for the global violation.

• global_exception: Exception | None Returns the global exception, if any.

---

Class Observation

Holds detailed observation results per condition.

Fields

• count: int – Number of evaluations
• violation: bool – Whether a violation occurred
• first_violation_at: int – Trial number of the first violation
• exc: Exception | None – Exception that occurred
• fail_condition: Callable[..., bool] | None – Condition function that failed
• fail_reason: str – Reason for the violation

---

Class ConditionStat

Simplified statistical representation of condition results.

Constructor

ConditionStat(count: int, violation: bool, first_violation_at: int)

Properties

• count: int – Number of evaluations
• violation: bool – Whether a violation occurred
• first_violation_at: int – Trial number of the first violation

---

Enum ExceptionKind

Indicates where an exception occurred.

Constants

• ON_CONDITION – Exception during condition evaluation
• ON_VIOLATION – Exception during violation handler execution
• ON_INTERNAL – Exception during internal processing

---

Testing

This module uses pytest for testing. Tests are located in the tests/ directory. The legacy/ directory contains disabled tests and should be skipped.