Pykka

Major feature release.

Dependencies

- Drop support for Python 2.6, 3.2, 3.3, and 3.4. All have reached their end of life and do no longer receive security updates.

- Include CPython 3.5, 3.6, 3.7, and 3.8 pre-releases, and PyPy 3.5 in the test matrix.

- Include gevent and Eventlet tests in all environments. Since Pykka was originally developed, both have grown support for Python 3 and PyPy.

- On Python 3, import `Callable` and `Iterable` from `collections.abc` instead of `collections`. This fixes a deprecation warning on Python 3.7 and prepares for Python 3.8.

Actors

- Actor messages are no longer required to be `dict` objects. Any object type can be used as an actor message. (Fixes: 39, 45, PR: 79)

For existing code, this means that `pykka.Actor.on_receive()` implementations should no longer assume the received message to be a
`dict`, and guard with the appropriate amount of `isinstance()` checks. As an existing application will not observe any new message types before it starts using them itself, this is not marked as backward incompatible.

Proxies

- **Backwards incompatible:** Avoid accessing actor properties when creating a proxy for the actor. For properties with side effects, this is a major bug fix. For properties which do heavy work, this is a major startup performance improvement.

This is backward incompatible if you in a property getter returned an object instance with the ``pykka_traversable`` marker. Previously, this would work just like a traversable attribute. Now, the property always returns a future with the property getter's return value.

- Fix infinite recursion when creating a proxy for an actor with an attribute or method replaced with a `unittest.mock.Mock` without a `spec` defined. (Fixes: 26, 27)

- Fix infinite recursion when creating a proxy for an actor with an attribute that was itself a proxy to the same actor. The attribute will now be ignored and a warning log message will ask you to consider making the self-proxy private. (Fixes: 48)

- Add `pykka.CallableProxy.defer()` to support method calls through a proxy with `pykka.ActorRef.tell()` semantics. (Contributed by Andrey Gubarev. Fixes: 63. PR: 72)

- Add `pykka.traversable()` for marking an actor's attributes as traversable when used through actor proxies. The old way of manually adding a `pykka_traversable` attribute to the object to be traversed still works, but the new function is recommended as it provides protection against typos in the marker name, and keeps the traversable marking in the actor class itself. (PR: 81)

Futures

- **Backwards incompatible:** `pykka.Future.set_exception()` no longer accepts an exception instance, which was deprecated in 0.15. The method can be called with either an `exc_info` tuple or `None`, in which case it will use `sys.exc_info()` to get information on the current exception.

- **Backwards incompatible:** `pykka.Future.map()` on a future with an iterable result no longer applies the map function to each item in iterable. Instead, the entire future result is passed to the map function. (Fixes: :issue:`64`)

To upgrade existing code, make sure to explicitly apply the core of your map function to each item in the iterable:

python
>>> f = pykka.ThreadingFuture()
>>> f.set([1, 2, 3])
>>> f.map(lambda x: x + 1).get() Pykka < 2.0
[2, 3, 4]
>>> f.map(lambda x: [i + 1 for i in x]).get() Pykka >= 2.0
[2, 3, 4]


This change makes it easy to use :meth:`pykka.Future.map` to extract a field from a future that returns a dict:

python
>>> f = pykka.ThreadingFuture()
>>> f.set({'foo': 'bar'})
>>> f.map(lambda x: x['foo']).get()
'bar'


Because `dict` is an iterable, the now removed special handling of iterables made this pattern difficult to use.

- Reuse result from `pykka.Future.filter()`, `pykka.Future.map()`, and `pykka.Future.reduce()`. Recalculating the result on each call to `pykka.Future.get()` is both inconsistent with regular futures and can cause problems if the function is expensive or has side effects. (Fixes: 32)

- If using Python 3.5+, one can now use the `await` keyword to get the result from a future. (Contributed by Joshua Doncaster-Marsiglio. PR: 78)

Logging

- Pykka's use of different log levels has been documented.

- Exceptions raised by an actor that are captured into a reply future are now logged on the `logging.INFO` level instead of the `logging.DEBUG` level. This makes it possible to detect potentially unhandled exceptions during development without having to turn on debug logging, which can have a low signal-to-noise ratio. (Contributed by Stefan Möhl. Fixes: 73)

Gevent support

- Ensure that the original traceback is preserved when an exception is returned through a future from a Gevent actor. (Contributed by Arne Brutschy. Fixes: 74, PR: 75)

Internals

- **Backwards incompatible:** Prefix all internal modules with `_`. This is backward-incompatible if you have imported objects from other import paths than what is used in the documentation.

- Port tests to pytest.

- Format code with Black.

- Change internal messaging format from `dict` to `namedtuple`. (PR: 80)

- Increase log level of `pykka.debug.log_thread_tracebacks()` debugging helper from `logging.INFO` to `logging.CRITICAL`.

- Fix errors in docs examples. (PR: 29, 43)

- Fix typos in docs.

- Various project setup and development improvements.

- Enforce that multiple calls to `pykka.Future.set()` raises an exception. This was already the case for some implementations. The exception raised is not specified.

- Add `pykka.Future.set_get_hook()`.

- Add `pykka.Future.filter()`, `pykka.Future.join()`, `pykka.Future.map()`, and `pykka.Future.reduce()` as convenience methods using the new `pykka.Future.set_get_hook()` method.

- Add support for running actors based on eventlet greenlets. See `pykka.eventlet` for details. Thanks to Jakub Stasiak for the implementation.

- Update documentation to reflect that the `reply_to` field on the message is private to Pykka. Actors should reply to messages simply by returning the response from `pykka.Actor.on_receive()`. The internal field is renamed to `pykka_reply_to` to avoid collisions with other message fields. It is also removed from the message before the message is passed to `pykka.Actor.on_receive()`. Thanks to Jakub Stasiak.

- When messages are left in the actor inbox after the actor is stopped, those messages that are expecting a reply are now rejected by replying with a `pykka.ActorDeadError` exception. This causes other actors that are blocking on the returned `pykka.Future` without a timeout to raise the exception instead of waiting forever. Thanks to Jakub Stasiak.

This makes the behavior of messaging an actor around the time it is stopped more consistent:

- Messaging an already dead actor immediately raises `pykka.ActorDeadError`.

- Messaging an alive actor that is stopped before it processes the message will cause the reply future to raise `pykka.ActorDeadError`.

Similarly, if you ask an actor to stop multiple times, and block on the responses, all the messages will now get a reply. Previously only the first message got a reply, potentially making the application wait forever on replies to the subsequent stop messages.

- When `pykka.ActorRef.ask()` is used to asynchronously message a dead actor (e.g. `block=False`), it will no longer immediately raise `pykka.ActorDeadError`. Instead, it will return a future and fail the future with the `pykka.ActorDeadError` exception. This makes the interface more consistent, as you'll have one instead of two ways the call can raise exceptions under normal conditions. If `pykka.ActorRef.ask` is called synchronously (e.g. `block=True`), the behavior is unchanged.

- A change to `pykka.ActorRef.stop()` reduces the likelihood of a race condition when asking an actor to stop multiple times by not checking if the actor is dead before asking it to stop, but instead, just go ahead and leave it to `pykka.ActorRef.tell()` to do the alive-or-dead check a single time, and as late as possible.

- Change `pykka.ActorRef.is_alive()` to check the actor's runnable flag instead of checking if the actor is registered in the actor registry.

- An exception raised in `pykka.Actor.on_start()` didn't stop the actor properly. Thanks to Jay Camp for finding and fixing the bug.

- Make sure exceptions in `pykka.Actor.on_stop()` and `pykka.Actor.on_failure()` is logged.

- Add `pykka.ThreadingActor.use_daemon_thread` flag for optionally running an actor on a daemon thread, so that it doesn't block the Python program from exiting. (Fixes: 14)

- Add `pykka.debug.log_thread_tracebacks()` debugging helper. (Fixes: 17)

- Name the threads of `pykka.ThreadingActor` after the actor class name instead of `PykkaThreadingActor-N` to ease debugging. (Fixes: 12)

- **Backwards incompatible:** Removed `pykka.VERSION` and `pykka.get_version()`, which have been deprecated since v0.14. Use `pykka.__version__` instead.

- **Backwards incompatible:** Removed `pykka.ActorRef.send_one_way()` and `pykka.ActorRef.send_request_reply()`, which have been deprecated since v0.14. Use `pykka.ActorRef.tell()` and `pykka.ActorRef.ask()` instead.

- **Backwards incompatible:** Actors no longer subclass `threading.Thread` or `gevent.Greenlet`. Instead, they *have* a thread or greenlet that executes the actor's main loop.

This is backward incompatible because you no longer have access to fields/methods of the thread/greenlet that runs the actor through fields/methods on the actor itself. This was never advertised in Pykka's docs or examples, but the fields/methods have always been available.

As a positive side effect, this fixes an issue on Python 3.x, that was introduced in Pykka 0.16, where `pykka.ThreadingActor` would
accidentally override the method `threading.Thread._stop()`.

- **Backwards incompatible:** Actors that override `pykka.Actor.__init__()` *must* call the method they override. If not, the actor will no longer be properly initialized. Valid ways to call the overridden `__init__()` method include::

python
super().__init__()
or
pykka.ThreadingActor.__init__()
or
pykka.gevent.GeventActor.__init__()


- Make `pykka.Actor.__init__()` accept any arguments and keyword arguments by default. This allows you to use `super()` in `__init__()` like this:

python
super().__init__(1, 2, 3, foo='bar')


Without this fix, the above use of `super()` would cause an exception because the default implementation of `__init__()` in `pykka.Actor` would not accept the arguments.

- Allow all public classes and functions to be imported directly from the `pykka` module. E.g. `from pykka.actor import ThreadingActor` can now be written as `from pykka import ThreadingActor`. The exception is `pykka.gevent`, which still needs to be imported from its own package due to its additional dependency on gevent.