# 17.6 `asynchat` -- Asynchronous socket command/response handler

This module builds on the `asyncore` infrastructure, simplifying asynchronous clients and servers and making it easier to handle protocols whose elements are terminated by arbitrary strings, or are of variable length. `asynchat` defines the abstract class `async_chat` that you subclass, providing implementations of the `collect_incoming_data()` and `found_terminator()` methods. It uses the same asynchronous loop as `asyncore`, and the two types of channel, `asyncore.dispatcher` and `asynchat.async_chat`, can freely be mixed in the channel map. Typically an `asyncore.dispatcher` server channel generates new `asynchat.async_chat` channel objects as it receives incoming connection requests.

class `async_chat`( | )

  This class is an abstract subclass of `asyncore.dispatcher`. To make practical use of the code you must subclass `async_chat`, providing meaningful `collect_incoming_data()` and `found_terminator()` methods. The `asyncore.dispatcher` methods can be used, although not all make sense in a message/response context.

  Like `asyncore.dispatcher`, `async_chat` defines a set of events that are generated by an analysis of socket conditions after a `select()` call. Once the polling loop has been started the `async_chat` object's methods are called by the event-processing framework with no action on the part of the programmer.

  Unlike `asyncore.dispatcher`, `async_chat` allows you to define a first-in-first-out queue (fifo) of producers. A producer need have only one method, `more()`, which should return data to be transmitted on the channel. The producer indicates exhaustion (i.e. that it contains no more data) by having its `more()` method return the empty string. At this point the `async_chat` object removes the producer from the fifo and starts using the next producer, if any. When the producer fifo is empty the `handle_write()` method does nothing. You use the channel object's `set_terminator()` method to describe how to recognize the end of, or an important breakpoint in, an incoming transmission from the remote endpoint.

  To build a functioning `async_chat` subclass your input methods `collect_incoming_data()` and `found_terminator()` must handle the data that the channel receives asynchronously. The methods are described below.

`close_when_done`( | )

  Pushes a `None` on to the producer fifo. When this producer is popped off the fifo it causes the channel to be closed.

`collect_incoming_data`( | data)

  Called with data holding an arbitrary amount of received data. The default method, which must be overridden, raises a `NotImplementedError` exception.

`discard_buffers`( | )

  In emergencies this method will discard any data held in the input and/or output buffers and the producer fifo.

`found_terminator`( | )

  Called when the incoming data stream matches the termination condition set by `set_terminator`. The default method, which must be overridden, raises a `NotImplementedError` exception. The buffered input data should be available via an instance attribute.

`get_terminator`( | )

  Returns the current terminator for the channel.

`handle_close`( | )

  Called when the channel is closed. The default method silently closes the channel's socket.

`handle_read`( | )

  Called when a read event fires on the channel's socket in the asynchronous loop. The default method checks for the termination condition established by `set_terminator()`, which can be either the appearance of a particular string in the input stream or the receipt of a particular number of characters. When the terminator is found, `handle_read` calls the `found_terminator()` method after calling `collect_incoming_data()` with any data preceding the terminating condition.

`handle_write`( | )

  Called when the application may write data to the channel. The default method calls the `initiate_send()` method, which in turn will call `refill_buffer()` to collect data from the producer fifo associated with the channel.

`push`( | data)

  Creates a `simple_producer` object (see below) containing the data and pushes it on to the channel's `producer_fifo` to ensure its transmission. This is all you need to do to have the channel write the data out to the network, although it is possible to use your own producers in more complex schemes to implement encryption and chunking, for example.

`push_with_producer`( | producer)

  Takes a producer object and adds it to the producer fifo associated with the channel. When all currently-pushed producers have been exhausted the channel will consume this producer's data by calling its `more()` method and send the data to the remote endpoint.

`readable`( | )

  Should return `True` for the channel to be included in the set of channels tested by the `select()` loop for readability.

`refill_buffer`( | )

  Refills the output buffer by calling the `more()` method of the producer at the head of the fifo. If it is exhausted then the producer is popped off the fifo and the next producer is activated. If the current producer is, or becomes, `None` then the channel is closed.

`set_terminator`( | term)

  Sets the terminating condition to be recognised on the channel. `term` may be any of three types of value, corresponding to three different ways to handle incoming protocol data.

  term | Description
  --- | ---
  string | Will call `found_terminator()` when the string is found in the input stream
  integer | Will call `found_terminator()` when the indicated number of characters have been received
  `None` | The channel continues to collect data forever

  Note that any data following the terminator will be available for reading by the channel after `found_terminator()` is called.

`writable`( | )

  Should return `True` as long as items remain on the producer fifo, or the channel is connected and the channel's output buffer is non-empty.