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17.5.

popen2

— Subprocesses with accessible

I/O streams

Deprecated since version 2.6: This module is obsolete. Use the

subprocess

module.

Check especially the Replacing Older Functions with the subprocess Module section.

This module allows you to spawn processes and connect to their input/output/error pipes

and obtain their return codes under Unix and Windows.

The

subprocess

module provides more powerful facilities for spawning new processes

and retrieving their results. Using the

subprocess

module is preferable to using the

popen2

module.

The primary interface offered by this module is a trio of factory functions. For each of

these, if bufsize is specified, it specifies the buffer size for the I/O pipes. mode, if

provided, should be the string

'b'

't'

; on Windows this is needed to determine

whether the file objects should be opened in binary or text mode. The default value for

mode is

't'

On Unix, cmd may be a sequence, in which case arguments will be passed directly to the

program without shell intervention (as with

os.spawnv()

). If cmd is a string it will be

passed to the shell (as with

os.system()

The only way to retrieve the return codes for the child processes is by using the

poll()

wait()

methods on the

Popen3

and

Popen4

classes; these are only available on Unix.

This information is not available when using the

popen2()

popen3()

, and

popen4()

functions, or the equivalent functions in the

module. (Note that the tuples returned by

the

module’s functions are in a different order from the ones returned by the

popen2

module.)

popen2.

popen2

(

cmd

[

, bufsize

[

, mode

]]

)

Executes cmd as a sub-process. Returns the file objects

(child_stdout,

child_stdin)

popen2.

popen3

(

cmd

[

, bufsize

[

, mode

]]

)

Executes cmd as a sub-process. Returns the file objects

(child_stdout,

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child_stdin, child_stderr)

popen2.

popen4

(

cmd

[

, bufsize

[

, mode

]]

)

Executes cmd as a sub-process. Returns the file objects

(child_stdout_and_stderr,

child_stdin)

New in version 2.0.

On Unix, a class defining the objects returned by the factory functions is also available.

These are not used for the Windows implementation, and are not available on that

platform.

class

popen2.

Popen3

(

cmd

[

, capturestderr

[

, bufsize

]]

)

This class represents a child process. Normally,

Popen3

instances are created using

the

popen2()

and

popen3()

factory functions described above.

If not using one of the helper functions to create

Popen3

objects, the parameter cmd is

the shell command to execute in a sub-process. The capturestderr flag, if true,

specifies that the object should capture standard error output of the child process.

The default is false. If the bufsize parameter is specified, it specifies the size of the

I/O buffers to/from the child process.

class

popen2.

Popen4

(

cmd

[

, bufsize

]

)

Similar to

Popen3

, but always captures standard error into the same file object as

standard output. These are typically created using

popen4()

New in version 2.0.

17.5.1. Popen3 and Popen4 Objects

Instances of the

Popen3

and

Popen4

classes have the following methods:

Popen3.

poll

()

Returns

-1

if child process hasn’t completed yet, or its status code (see

wait()

)

otherwise.

Popen3.

wait

()

Waits for and returns the status code of the child process. The status code encodes

both the return code of the process and information about whether it exited using the

exit()

system call or died due to a signal. Functions to help interpret the status code

are defined in the

module; see section Process Management for the

W*()

family of

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functions.

The following attributes are also available:

Popen3.

fromchild

A file object that provides output from the child process. For

Popen4

instances, this will

provide both the standard output and standard error streams.

Popen3.

tochild

A file object that provides input to the child process.

Popen3.

childerr

A file object that provides error output from the child process, if capturestderr was

true for the constructor, otherwise

None

. This will always be

None

for

Popen4

instances.

Popen3.

pid

The process ID of the child process.

17.5.2. Flow Control Issues

Any time you are working with any form of inter-process communication, control flow

needs to be carefully thought out. This remains the case with the file objects provided by

this module (or the

module equivalents).

When reading output from a child process that writes a lot of data to standard error while

the parent is reading from the child’s standard output, a deadlock can occur. A similar

situation can occur with other combinations of reads and writes. The essential factors are

that more than

_PC_PIPE_BUF

bytes are being written by one process in a blocking

fashion, while the other process is reading from the first process, also in a blocking

fashion.

There are several ways to deal with this situation.

The simplest application change, in many cases, will be to follow this model in the parent

process:

import popen2

r, w, e = popen2.popen3('python slave.py')

e.readlines()

r.readlines()

r.close()

e.close()

w.close()

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with code like this in the child:

import os

import sys

# note that each of these print statements

# writes a single long string

print >>sys.stderr, 400 * 'this is a test\n'

os.close(sys.stderr.fileno())

print >>sys.stdout, 400 * 'this is another test\n'

In particular, note that

sys.stderr

must be closed after writing all data, or

readlines()

won’t return. Also note that

os.close()

must be used, as

sys.stderr.close()

won’t

stderr

(otherwise assigning to

sys.stderr

will silently close it, so no further errors

can be printed).

Applications which need to support a more general approach should integrate I/O over

pipes with their

select()

loops, or use separate threads to read each of the individual

files provided by whichever

popen*()

function or

Popen*

class was used.

See also:

Module

subprocess

Module for spawning and managing subprocesses.

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17.6.

asyncore

— Asynchronous socket handler

Source code: Lib/asyncore.py

This module provides the basic infrastructure for writing asynchronous socket service

clients and servers.

There are only two ways to have a program on a single processor do “more than one

thing at a time.” Multi-threaded programming is the simplest and most popular way to do

it, but there is another very different technique, that lets you have nearly all the

advantages of multi-threading, without actually using multiple threads. It’s really only

practical if your program is largely I/O bound. If your program is processor bound, then

pre-emptive scheduled threads are probably what you really need. Network servers are

rarely processor bound, however.

If your operating system supports the

select()

system call in its I/O library (and nearly

all do), then you can use it to juggle multiple communication channels at once; doing

other work while your I/O is taking place in the “background.” Although this strategy can

seem strange and complex, especially at first, it is in many ways easier to understand

and control than multi-threaded programming. The

asyncore

module solves many of the

difficult problems for you, making the task of building sophisticated high-performance

network servers and clients a snap. For “conversational” applications and protocols the

companion

asynchat

module is invaluable.

The basic idea behind both modules is to create one or more network channels,

instances of class

asyncore.dispatcher

and

asynchat.async_chat

. Creating the

channels adds them to a global map, used by the

loop()

function if you do not provide it

with your own map.

Once the initial channel(s) is(are) created, calling the

loop()

function activates channel

service, which continues until the last channel (including any that have been added to the

map during asynchronous service) is closed.

asyncore.

loop

(

[

timeout

[

, use_poll

[

, map

[

, count

]]]]

)

Enter a polling loop that terminates after count passes or all open channels have been

closed. All arguments are optional. The count parameter defaults to None, resulting in

the loop terminating only when all channels have been closed. The timeout argument

sets the timeout parameter for the appropriate

select()

poll()

call, measured in

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seconds; the default is 30 seconds. The use_poll parameter, if true, indicates that

poll()

should be used in preference to

select()

(the default is

False

The map parameter is a dictionary whose items are the channels to watch. As

channels are closed they are deleted from their map. If map is omitted, a global map

is used. Channels (instances of

asyncore.dispatcher

asynchat.async_chat

and

subclasses thereof) can freely be mixed in the map.

class

asyncore.

dispatcher

The

dispatcher

class is a thin wrapper around a low-level socket object. To make it

more useful, it has a few methods for event-handling which are called from the

asynchronous loop. Otherwise, it can be treated as a normal non-blocking socket

object.

The firing of low-level events at certain times or in certain connection states tells the

asynchronous loop that certain higher-level events have taken place. For example, if

we have asked for a socket to connect to another host, we know that the connection

has been made when the socket becomes writable for the first time (at this point you

know that you may write to it with the expectation of success). The implied higher-

level events are:

Event

Description

handle_connect()

Implied by the first read or write event

handle_close()

Implied by a read event with no data

available

handle_accept()

Implied by a read event on a listening socket

During asynchronous processing, each mapped channel’s

readable()

and

writable()

methods are used to determine whether the channel’s socket should be

added to the list of channels

select()

ed or

poll()

ed for read and write events.

Thus, the set of channel events is larger than the basic socket events. The full set of

methods that can be overridden in your subclass follows:

handle_read

()

Called when the asynchronous loop detects that a

read()

call on the channel’s

socket will succeed.

handle_write

()

Called when the asynchronous loop detects that a writable socket can be written.

Often this method will implement the necessary buffering for performance. For

example:

def handle_write(self):

sent = self.send(self.buffer)

self.buffer = self.buffer[sent:]

handle_expt

()

Called when there is out of band (OOB) data for a socket connection. This will

almost never happen, as OOB is tenuously supported and rarely used.

handle_connect

()

Called when the active opener’s socket actually makes a connection. Might send a

“welcome” banner, or initiate a protocol negotiation with the remote endpoint, for

example.

handle_close

()

Called when the socket is closed.

handle_error

()

Called when an exception is raised and not otherwise handled. The default version

prints a condensed traceback.

handle_accept

()

Called on listening channels (passive openers) when a connection can be

established with a new remote endpoint that has issued a

connect()

call for the

local endpoint.

readable

()

Called each time around the asynchronous loop to determine whether a channel’s

socket should be added to the list on which read events can occur. The default

method simply returns

True

, indicating that by default, all channels will be

interested in read events.

writable

()

Called each time around the asynchronous loop to determine whether a channel’s

socket should be added to the list on which write events can occur. The default

method simply returns

True

, indicating that by default, all channels will be

interested in write events.

In addition, each channel delegates or extends many of the socket methods. Most of

these are nearly identical to their socket partners.

create_socket

(

family, type

)

This is identical to the creation of a normal socket, and will use the same options

for creation. Refer to the

socket

documentation for information on creating

sockets.

connect

(

address

)

As with the normal socket object, address is a tuple with the first element the host

to connect to, and the second the port number.

send

(

data

)

Send data to the remote end-point of the socket.

recv

(

buffer_size

)

Read at most buffer_size bytes from the socket’s remote end-point. An empty

string implies that the channel has been closed from the other end.

listen

(

backlog

)

Listen for connections made to the socket. The backlog argument specifies the

maximum number of queued connections and should be at least 1; the maximum

value is system-dependent (usually 5).

bind

(

address

)

Bind the socket to address. The socket must not already be bound. (The format of

address depends on the address family — refer to the

socket

documentation for

more information.) To mark the socket as re-usable (setting the

SO_REUSEADDR

option), call the

dispatcher

object’s

set_reuse_addr()

method.

()

Accept a connection. The socket must be bound to an address and listening for

connections. The return value can be either

None

or a pair

(conn, address)

where

conn is a new socket object usable to send and receive data on the connection,

and address is the address bound to the socket on the other end of the

connection. When

None

is returned it means the connection didn’t take place, in

which case the server should just ignore this event and keep listening for further

incoming connections.

()

Close the socket. All future operations on the socket object will fail. The remote

end-point will receive no more data (after queued data is flushed). Sockets are

automatically closed when they are garbage-collected.

class

asyncore.

dispatcher_with_send

dispatcher

subclass which adds simple buffered output capability, useful for simple

clients. For more sophisticated usage use

asynchat.async_chat

class

asyncore.

file_dispatcher

A file_dispatcher takes a file descriptor or file object along with an optional map

argument and wraps it for use with the

poll()

loop()

functions. If provided a file

object or anything with a

fileno()

method, that method will be called and passed to

the

file_wrapper

constructor. Availability: UNIX.

class

asyncore.

file_wrapper

A file_wrapper takes an integer file descriptor and calls

os.dup()

to duplicate the

handle so that the original handle may be closed independently of the file_wrapper.

This class implements sufficient methods to emulate a socket for use by the

file_dispatcher

class. Availability: UNIX.

17.6.1. asyncore Example basic HTTP client

Here is a very basic HTTP client that uses the

dispatcher

class to implement its socket

handling:

import asyncore, socket

class HTTPClient(asyncore.dispatcher):

def __init__(self, host, path):

asyncore.dispatcher.__init__(self)

self.create_socket(socket.AF_INET, socket.SOCK_STREAM)

self.connect( (host, 80) )

self.buffer = 'GET %s HTTP/1.0\r\n\r\n' % path

def handle_connect(self):

pass

def handle_close(self):

self.close()

def handle_read(self):

print self.recv(8192)

def writable(self):

return (len(self.buffer) > 0)

def handle_write(self):

sent = self.send(self.buffer)

self.buffer = self.buffer[sent:]

client = HTTPClient('www.python.org', '/')

asyncore.loop()

17.6.2. asyncore Example basic echo server

Here is a basic echo server that uses the

dispatcher

class to accept connections and

dispatches the incoming connections to a handler:

import asyncore

import socket

class EchoHandler(asyncore.dispatcher_with_send):

def handle_read(self):

data = self.recv(8192)

if data:

self.send(data)

class EchoServer(asyncore.dispatcher):

def __init__(self, host, port):

asyncore.dispatcher.__init__(self)

self.create_socket(socket.AF_INET, socket.SOCK_STREAM)

self.set_reuse_addr()

self.bind((host, port))

self.listen(5)

def handle_accept(self):

pair = self.accept()

if pair is not None:

sock, addr = pair

print 'Incoming connection from %s' % repr(addr)

handler = EchoHandler(sock)

server = EchoServer('localhost', 8080)

asyncore.loop()

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Python » Python v2.7.6 documentation » The Python Standard Library » 17. Interprocess

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