\input{libasynchat}

\section{\module{asynchat} ---

Asynchronous socket command/response handler}

\declaremodule{standard}{asynchat}

\modulesynopsis{Support for asynchronous command/response protocols.}

\moduleauthor{Sam Rushing}{[email protected]}

\sectionauthor{Steve Holden}{[email protected]}

This module builds on the \refmodule{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. \refmodule{asynchat} defines the abstract class

\class{async_chat} that you subclass, providing implementations of the

\method{collect_incoming_data()} and \method{found_terminator()}

methods. It uses the same asynchronous loop as \refmodule{asyncore}, and

the two types of channel, \class{asyncore.despatcher} and

\class{asynchat.async_chat}, can freely be mixed in the channel map.

Typically an \class{asyncore.despatcher} server channel generates new

\class{asynchat.async_chat} channel objects as it receives incoming

connection requests.

\begin{classdesc}{async_chat}{}

This class is an abstract subclass of \class{asyncore.despatcher}. To make

practical use of the code you must subclass \class{async_chat}, providing

meaningful \method{collect_incoming_data()} and \method{found_terminator()}

methods. The \class{asyncore.despatcher} methods can be

used, although not all make sense in a message/response context.

Like \class{asyncore.despatcher}, \class{async_chat} defines a set of events

that are generated by an analysis of socket conditions after a

\cfunction{select()} call. Once the polling loop has been started the

\class{async_chat} object's methods are called by the event-processing

framework with no action on the part of the programmer.

Unlike \class{asyncore.despatcher}, \class{async_chat} allows you to define

a first-in-first-out queue (fifo) of \emph{producers}. A producer need have

only one method, \method{more()}, which should return data to be transmitted

on the channel. The producer indicates exhaustion (\emph{i.e.} that it contains

no more data) by having its \method{more()} method return the empty string. At

this point the \class{async_chat} object removes the producer from the fifo

and starts using the next producer, if any. When the producer fifo is empty

the \method{handle_write()} method does nothing. You use the channel object's

\method{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 \class{async_chat} subclass your

input methods \method{collect_incoming_data()} and

\method{found_terminator()} must handle the data that the channel receives

asynchronously. The methods are described below.

\end{classdesc}

\begin{methoddesc}{close_when_done}{}

Pushes a \code{None} on to the producer fifo. When this producer is

popped off the fifo it causes the channel to be closed.

\end{methoddesc}

\begin{methoddesc}{collect_incoming_data}{data}

Called with \var{data} holding an arbitrary amount of received data.

The default method, which must be overridden, raises a \exception{NotImplementedError} exception.

\end{methoddesc}

\begin{methoddesc}{discard_buffers}{}

In emergencies this method will discard any data held in the input and/or

output buffers and the producer fifo.

\end{methoddesc}

\begin{methoddesc}{found_terminator}{}

Called when the incoming data stream matches the termination condition

set by \method{set_terminator}. The default method, which must be overridden,

raises a \exception{NotImplementedError} exception. The buffered input data should

be available via an instance attribute.

\end{methoddesc}

\begin{methoddesc}{get_terminator}{}

Returns the current terminator for the channel.

\end{methoddesc}

\begin{methoddesc}{handle_close}{}

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

the channel's socket.

\end{methoddesc}

\begin{methoddesc}{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 \method{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,

\method{handle_read} calls the \method{found_terminator()} method after

calling \method{collect_incoming_data()} with any data preceding the

terminating condition.

\end{methoddesc}

\begin{methoddesc}{handle_write}{}

Called when the application may write data to the channel.

The default method calls the \method{initiate_send()} method, which in turn

will call \method{refill_buffer()} to collect data from the producer

fifo associated with the channel.

\end{methoddesc}

\begin{methoddesc}{push}{data}

Creates a \class{simple_producer} object (\emph{see below}) containing the data and

pushes it on to the channel's \code{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.

\end{methoddesc}

\begin{methoddesc}{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

\method{more()} method and send the data to the remote endpoint.

\end{methoddesc}

\begin{methoddesc}{readable}{}

Should return \code{True} for the channel to be included in the set of

channels tested by the \cfunction{select()} loop for readability.

\end{methoddesc}

\begin{methoddesc}{refill_buffer}{}

Refills the output buffer by calling the \method{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, \code{None} then the channel

is closed.

\end{methoddesc}

\begin{methoddesc}{set_terminator}{term}

Sets the terminating condition to be recognised on the channel. \code{term}

may be any of three types of value, corresponding to three different ways

to handle incoming protocol data.

\begin{tableii}{l|l}{}{term}{Description}

\lineii{\emph{string}}{Will call \method{found_terminator()} when the

string is found in the input stream}

\lineii{\emph{integer}}{Will call \method{found_terminator()} when the

indicated number of characters have been received}

\lineii{\code{None}}{The channel continues to collect data forever}

\end{tableii}

Note that any data following the terminator will be available for reading by

the channel after \method{found_terminator()} is called.

\end{methoddesc}

\begin{methoddesc}{writable}{}

Should return \code{True} as long as items remain on the producer fifo,

or the channel is connected and the channel's output buffer is non-empty.

\end{methoddesc}

\subsection{asynchat - Auxiliary Classes and Functions}

\begin{classdesc}{simple_producer}{data\optional{, buffer_size=512}}

A \class{simple_producer} takes a chunk of data and an optional buffer size.

Repeated calls to its \method{more()} method yield successive chunks of the

data no larger than \var{buffer_size}.

\end{classdesc}

\begin{methoddesc}{more}{}

Produces the next chunk of information from the producer, or returns the empty string.

\end{methoddesc}

\begin{classdesc}{fifo}{\optional{list=None}}

Each channel maintains a \class{fifo} holding data which has been pushed by the

application but not yet popped for writing to the channel.

A \class{fifo} is a list used to hold data and/or producers until they are required.

If the \var{list} argument is provided then it should contain producers or

data items to be written to the channel.

\end{classdesc}

\begin{methoddesc}{is_empty}{}

Returns \code{True} iff the fifo is empty.

\end{methoddesc}

\begin{methoddesc}{first}{}

Returns the least-recently \method{push()}ed item from the fifo.

\end{methoddesc}

\begin{methoddesc}{push}{data}

Adds the given data (which may be a string or a producer object) to the

producer fifo.

\end{methoddesc}

\begin{methoddesc}{pop}{}

If the fifo is not empty, returns \code{True, first()}, deleting the popped

item. Returns \code{False, None} for an empty fifo.

\end{methoddesc}

The \module{asynchat} module also defines one utility function, which may be

of use in network and textual analysis operations.

\begin{funcdesc}{find_prefix_at_end}{haystack, needle}

Returns \code{True} if string \var{haystack} ends with any non-empty

prefix of string \var{needle}.

\end{funcdesc}

\subsection{asynchat Example \label{asynchat-example}}

The following partial example shows how HTTP requests can be read with

\class{async_chat}. A web server might create an \class{http_request_handler} object for

each incoming client connection. Notice that initially the

channel terminator is set to match the blank line at the end of the HTTP

headers, and a flag indicates that the headers are being read.

Once the headers have been read, if the request is of type POST

(indicating that further data are present in the input stream) then the

\code{Content-Length:} header is used to set a numeric terminator to

read the right amount of data from the channel.

The \method{handle_request()} method is called once all relevant input

has been marshalled, after setting the channel terminator to \code{None}

to ensure that any extraneous data sent by the web client are ignored.

\begin{verbatim}

\end{verbatim}