.. _internals-guide: ================================ Contributors Guide to the Code ================================ .. contents:: :local: Philosophy ========== The API>RCP Precedence Rule --------------------------- - The API is more important than Readability - Readability is more important than Convention - Convention is more important than Performance - …unless the code is a proven hotspot. More important than anything else is the end-user API. Conventions must step aside, and any suffering is always alleviated if the end result is a better API. Conventions and Idioms Used =========================== Classes ------- Naming ~~~~~~ - Follows :pep:`8`. - Class names must be `CamelCase`. - but not if they are verbs, verbs shall be `lower_case`: .. code-block:: python # - test case for a class class TestMyClass(Case): # BAD pass class test_MyClass(Case): # GOOD pass # - test case for a function class TestMyFunction(Case): # BAD pass class test_my_function(Case): # GOOD pass # - "action" class (verb) class UpdateTwitterStatus(object): # BAD pass class update_twitter_status(object): # GOOD pass .. note:: Sometimes it makes sense to have a class mask as a function, and there is precedence for this in the stdlib (e.g. :class:`~contextlib.contextmanager`). Celery examples include :class:`~celery.subtask`, :class:`~celery.chord`, ``inspect``, :class:`~kombu.utils.functional.promise` and more.. - Factory functions and methods must be `CamelCase` (excluding verbs): .. code-block:: python class Celery(object): def consumer_factory(self): # BAD ... def Consumer(self): # GOOD ... Default values ~~~~~~~~~~~~~~ Class attributes serve as default values for the instance, as this means that they can be set by either instantiation or inheritance. **Example:** .. code-block:: python class Producer(object): active = True serializer = 'json' def __init__(self, serializer=None): self.serializer = serializer or self.serializer # must check for None when value can be false-y self.active = active if active is not None else self.active A subclass can change the default value: .. code-block:: python TaskProducer(Producer): serializer = 'pickle' and the value can be set at instantiation: .. code-block:: python >>> producer = TaskProducer(serializer='msgpack') Exceptions ~~~~~~~~~~ Custom exceptions raised by an objects methods and properties should be available as an attribute and documented in the method/property that throw. This way a user doesn't have to find out where to import the exception from, but rather use ``help(obj)`` and access the exception class from the instance directly. **Example**: .. code-block:: python class Empty(Exception): pass class Queue(object): Empty = Empty def get(self): """Get the next item from the queue. :raises Queue.Empty: if there are no more items left. """ try: return self.queue.popleft() except IndexError: raise self.Empty() Composites ~~~~~~~~~~ Similarly to exceptions, composite classes should be override-able by inheritance and/or instantiation. Common sense can be used when selecting what classes to include, but often it's better to add one too many: predicting what users need to override is hard (this has saved us from many a monkey patch). **Example**: .. code-block:: python class Worker(object): Consumer = Consumer def __init__(self, connection, consumer_cls=None): self.Consumer = consumer_cls or self.Consumer def do_work(self): with self.Consumer(self.connection) as consumer: self.connection.drain_events() Applications vs. "single mode" ============================== In the beginning Celery was developed for Django, simply because this enabled us get the project started quickly, while also having a large potential user base. In Django there is a global settings object, so multiple Django projects can't co-exist in the same process space, this later posed a problem for using Celery with frameworks that doesn't have this limitation. Therefore the app concept was introduced. When using apps you use 'celery' objects instead of importing things from celery submodules, this (unfortunately) also means that Celery essentially has two API's. Here's an example using Celery in single-mode: .. code-block:: python from celery import task from celery.task.control import inspect from .models import CeleryStats @task def write_stats_to_db(): stats = inspect().stats(timeout=1) for node_name, reply in stats: CeleryStats.objects.update_stat(node_name, stats) and here's the same using Celery app objects: .. code-block:: python from .celery import celery from .models import CeleryStats @app.task def write_stats_to_db(): stats = celery.control.inspect().stats(timeout=1) for node_name, reply in stats: CeleryStats.objects.update_stat(node_name, stats) In the example above the actual application instance is imported from a module in the project, this module could look something like this: .. code-block:: python from celery import Celery app = Celery(broker='amqp://') Module Overview =============== - celery.app This is the core of Celery: the entry-point for all functionality. - celery.loaders Every app must have a loader. The loader decides how configuration is read, what happens when the worker starts, when a task starts and ends, and so on. The loaders included are: - app Custom celery app instances uses this loader by default. - default "single-mode" uses this loader by default. Extension loaders also exist, like ``django-celery``, ``celery-pylons`` and so on. - celery.worker This is the worker implementation. - celery.backends Task result backends live here. - celery.apps Major user applications: worker and beat. The command-line wrappers for these are in celery.bin (see below) - celery.bin Command-line applications. setup.py creates setuptools entrypoints for these. - celery.concurrency Execution pool implementations (prefork, eventlet, gevent, threads). - celery.db Database models for the SQLAlchemy database result backend. (should be moved into :mod:`celery.backends.database`) - celery.events Sending and consuming monitoring events, also includes curses monitor, event dumper and utilities to work with in-memory cluster state. - celery.execute.trace How tasks are executed and traced by the worker, and in eager mode. - celery.security Security related functionality, currently a serializer using cryptographic digests. - celery.task single-mode interface to creating tasks, and controlling workers. - celery.tests The unittest suite. - celery.utils Utility functions used by the celery code base. Much of it is there to be compatible across Python versions. - celery.contrib Additional public code that doesn't fit into any other namespace.