Pika

Pika is a RabbitMQ (AMQP-0-9-1) client library for Python.

Introduction

Pika is a pure-Python implementation of the AMQP 0-9-1 protocol including RabbitMQ's extensions.

• Python 2.7 and 3.4+ are supported.
• Since threads aren't appropriate to every situation, it doesn't require threads. It takes care not to forbid them, either. The same goes for greenlets, callbacks, continuations and generators. It is not necessarily thread-safe however, and your mileage will vary.
• People may be using direct sockets, plain old select(), or any of the wide variety of ways of getting network events to and from a python application. Pika tries to stay compatible with all of these, and to make adapting it to a new environment as simple as possible.

Documentation

Pika's documentation can be found at https://pika.readthedocs.io

Example

Here is the most simple example of use, sending a message with the BlockingConnection adapter:

import pika
connection = pika.BlockingConnection()
channel = connection.channel()
channel.basic_publish(exchange='example',
                      routing_key='test',
                      body='Test Message')
connection.close()

And an example of writing a blocking consumer:

import pika
connection = pika.BlockingConnection()
channel = connection.channel()

for method_frame, properties, body in channel.consume('test'):

    # Display the message parts and ack the message
    print(method_frame, properties, body)
    channel.basic_ack(method_frame.delivery_tag)

    # Escape out of the loop after 10 messages
    if method_frame.delivery_tag == 10:
        break

# Cancel the consumer and return any pending messages
requeued_messages = channel.cancel()
print('Requeued %i messages' % requeued_messages)
connection.close()

Pika provides the following adapters

• AsyncioConnection - adapter for the Python3 AsyncIO event loop
• BlockingConnection - enables blocking, synchronous operation on top of library for simple uses
• SelectConnection - fast asynchronous adapter
• TornadoConnection - adapter for use with the Tornado IO Loop http://tornadoweb.org
• TwistedConnection - adapter for use with the Twisted asynchronous package http://twistedmatrix.com/

Requesting message ACKs from another thread

The single-threaded usage constraint of an individual Pika connection adapter instance may result in a dropped AMQP/stream connection due to AMQP heartbeat timeout in consumers that take a long time to process an incoming message. A common solution is to delegate processing of the incoming messages to another thread, while the connection adapter's thread continues to service its ioloop's message pump, permitting AMQP heartbeats and other I/O to be serviced in a timely fashion.

Messages processed in another thread may not be ACK'ed directly from that thread, since all accesses to the connection adapter instance must be from a single thread - the thread that is running the adapter's ioloop. However, this may be accomplished by requesting a callback to be executed in the adapter's ioloop thread. For example, the callback function's implementation might look like this:

def ack_message(channel, delivery_tag):
    """Note that `channel` must be the same pika channel instance via which
    the message being ACKed was retrieved (AMQP protocol constraint).
    """
    if channel.is_open:
        channel.basic_ack(delivery_tag)
    else:
        # Channel is already closed, so we can't ACK this message;
        # log and/or do something that makes sense for your app in this case.
        pass

The code running in the other thread may request the ack_message() function to be executed in the connection adapter's ioloop thread using an adapter-specific mechanism:

• :py:class:`pika.BlockingConnection` abstracts its ioloop from the application and thus exposes :py:meth:`pika.BlockingConnection.add_callback_threadsafe()`. Refer to this method's docstring for additional information. For example:

  connection.add_callback_threadsafe(functools.partial(ack_message, channel, delivery_tag))

• When using a non-blocking connection adapter, such as

:py:class:`pika.adapters.asyncio_connection.AsyncioConnection` or :py:class:`pika.SelectConnection`, you use the underlying asynchronous framework's native API for requesting an ioloop-bound callback from another thread. For example, SelectConnection's IOLoop provides add_callback_threadsafe(), Tornado's IOLoop has add_callback(), while asyncio's event loop exposes call_soon_threadsafe().

This threadsafe callback request mechanism may also be used to delegate publishing of messages, etc., from a background thread to the connection adapter's thread.

Contributing

To contribute to pika, please make sure that any new features or changes to existing functionality include test coverage.

Pull requests that add or change code without coverage will most likely be rejected.

Additionally, please format your code using yapf with google style prior to issuing your pull request. Note: only format those lines that you have changed in your pull request. If you format an entire file and change code outside of the scope of your PR, it will likely be rejected.