explain when to use a task queue

remove deprecated abstract attr from celery
add explanation of example task
[ci skip]

docs/patterns/celery.rst

@@ -1,24 +1,27 @@
-Celery Based Background Tasks
-=============================
+Celery Background Tasks
+=======================
 
-Celery is a task queue for Python with batteries included.  It used to
-have a Flask integration but it became unnecessary after some
-restructuring of the internals of Celery with Version 3.  This guide fills
-in the blanks in how to properly use Celery with Flask but assumes that
-you generally already read the `First Steps with Celery
-<http://docs.celeryproject.org/en/latest/getting-started/first-steps-with-celery.html>`_
-guide in the official Celery documentation.
+If your application has a long running task, such as processing some uploaded
+data or sending email, you don't want to wait for it to finish during a
+request. Instead, use a task queue to send the necessary data to another
+process that will run the task in the background while the request returns
+immediately.
 
-Installing Celery
------------------
+Celery is a powerful task queue that can be used for simple background tasks
+as well as complex multi-stage programs and schedules. This guide will show you
+how to configure Celery using Flask, but assumes you've already read the
+`First Steps with Celery <http://docs.celeryproject.org/en/latest/getting-started/first-steps-with-celery.html>`_
+guide in the Celery documentation.
 
-Celery is on the Python Package Index (PyPI), so it can be installed with
-standard Python tools like :command:`pip` or :command:`easy_install`::
+Install
+-------
+
+Celery is a separate Python package. Install it from PyPI using pip::
 
     $ pip install celery
 
-Configuring Celery
-------------------
+Configure
+---------
 
 The first thing you need is a Celery instance, this is called the celery
 application.  It serves the same purpose as the :class:`~flask.Flask`
@@ -36,15 +39,18 @@ This is all that is necessary to properly integrate Celery with Flask::
     from celery import Celery
 
     def make_celery(app):
-        celery = Celery(app.import_name, backend=app.config['CELERY_RESULT_BACKEND'],
-                        broker=app.config['CELERY_BROKER_URL'])
+        celery = Celery(
+            app.import_name,
+            backend=app.config['CELERY_RESULT_BACKEND'],
+            broker=app.config['CELERY_BROKER_URL']
+        )
         celery.conf.update(app.config)
-        TaskBase = celery.Task
-        class ContextTask(TaskBase):
-            abstract = True
+
+        class ContextTask(celery.Task):
             def __call__(self, *args, **kwargs):
                 with app.app_context():
                     return self.run(*args, **kwargs)
+
         celery.Task = ContextTask
         return celery
 
@@ -53,11 +59,12 @@ from the application config, updates the rest of the Celery config from
 the Flask config and then creates a subclass of the task that wraps the
 task execution in an application context.
 
-Minimal Example
+An example task
 ---------------
 
-With what we have above this is the minimal example of using Celery with
-Flask::
+Let's write a task that adds two numbers together and returns the result. We
+configure Celery's broker and backend to use Redis, create a ``celery``
+application using the factor from above, and then use it to define the task. ::
 
     from flask import Flask
 
@@ -68,26 +75,27 @@ Flask::
     )
     celery = make_celery(flask_app)
 
-
     @celery.task()
     def add_together(a, b):
         return a + b
 
-This task can now be called in the background:
+This task can now be called in the background::
 
->>> result = add_together.delay(23, 42)
->>> result.wait()
-65
+    result = add_together.delay(23, 42)
+    result.wait()  # 65
 
-Running the Celery Worker
--------------------------
+Run a worker
+------------
 
-Now if you jumped in and already executed the above code you will be
-disappointed to learn that your ``.wait()`` will never actually return.
-That's because you also need to run celery.  You can do that by running
-celery as a worker::
+If you jumped in and already executed the above code you will be
+disappointed to learn that ``.wait()`` will never actually return.
+That's because you also need to run a Celery worker to receive and execute the
+task. ::
 
     $ celery -A your_application.celery worker
 
 The ``your_application`` string has to point to your application's package
-or module that creates the `celery` object.
+or module that creates the ``celery`` object.
+
+Now that the worker is running, ``wait`` will return the result once the task
+is finished.