Along with connection to servers across the internet, Twisted also connects to local processes with much the same API. The API is described in more detail in the documentation of:
Processes are run through the reactor,
using reactor.spawnProcess . Pipes are created to the child process,
and added to the reactor core so that the application will not block while
sending data into or pulling data out of the new
process. reactor.spawnProcess requires two arguments,
processProtocol and executable , and optionally takes
several more: args , environment ,
path , userID , groupID ,
usePTY , and childFDs . Not all of these are
available on Windows.
from twisted.internet import reactor
processProtocol = MyProcessProtocol()
reactor.spawnProcess(processProtocol, executable, args=[program, arg1, arg2],
env={'HOME': os.environ['HOME']}, path,
uid, gid, usePTY, childFDs)
processProtocol should be an instance of a subclass of
twisted.internet.protocol.ProcessProtocol . The
interface is described below.
executable is the full path of the program to run. It
will be connected to processProtocol.
args is a list of command line arguments to be passed to
the process. args[0] should be the name of the process.
env is a dictionary containing the environment to pass
through to the process.
path is the directory to run the process in. The child
will switch to the given directory just before starting the new program.
The default is to stay in the current directory.
uid and gid are the user ID and group ID to
run the subprocess as. Of course, changing identities will be more likely
to succeed if you start as root.
usePTY specifies whether the child process should be run
with a pty, or if it should just get a pair of pipes. Whether a program
needs to be run with a PTY or not depends on the particulars of that
program. Often, programs which primarily interact with users via a terminal
do need a PTY.
childFDs lets you specify how the child’s file
descriptors should be set up. Each key is a file descriptor number (an
integer) as seen by the child. 0, 1, and 2 are usually stdin, stdout, and
stderr, but some programs may be instructed to use additional fds through
command-line arguments or environment variables. Each value is either an
integer specifying one of the parent’s current file descriptors, the
string “r” which creates a pipe that the parent can read from, or the
string “w” which creates a pipe that the parent can write to. If
childFDs is not provided, a default is used which creates the
usual stdin-writer, stdout-reader, and stderr-reader pipes.
args and env have empty default values, but
many programs depend upon them to be set correctly. At the very least,
args[0] should probably be the same as executable .
If you just provide os.environ for env , the child
program will inherit the environment from the current process, which is
usually the civilized thing to do (unless you want to explicitly clean the
environment as a security precaution). The default is to give an empty env to the child.
reactor.spawnProcess returns an instance that implements IProcessTransport.
The ProcessProtocol you pass to spawnProcess is your
interaction with the process. It has a very similar signature to a regular
Protocol, but it has several extra methods to deal with events specific to
a process. In our example, we will interface with ‘wc’ to create a word count
of user-given text. First, we’ll start by importing the required modules, and
writing the initialization for our ProcessProtocol.
from twisted.internet import protocol
class WCProcessProtocol(protocol.ProcessProtocol):
def __init__(self, text):
self.text = text
When the ProcessProtocol is connected to the protocol, it has the connectionMade method called. In our protocol, we will write our text to the standard input of our process and then close standard input, to let the process know we are done writing to it.
...
def connectionMade(self):
self.transport.write(self.text)
self.transport.closeStdin()
At this point, the process has received the data, and it’s time for us
to read the results. Instead of being received in dataReceived ,
data from standard output is received in outReceived . This is
to distinguish it from data on standard error.
...
def outReceived(self, data):
fieldLength = len(data) / 3
lines = int(data[:fieldLength])
words = int(data[fieldLength:fieldLength*2])
chars = int(data[fieldLength*2:])
self.transport.loseConnection()
self.receiveCounts(lines, words, chars)
Now, the process has parsed the output, and ended the connection to the process. Then it sends the results on to the final method, receiveCounts. This is for users of the class to override, so as to do other things with the data. For our demonstration, we will just print the results.
...
def receiveCounts(self, lines, words, chars):
print('Received counts from wc.')
print('Lines:', lines)
print('Words:', words)
print('Characters:', chars)
We’re done! To use our WCProcessProtocol, we create an instance, and pass it to spawnProcess.
from twisted.internet import reactor
wcProcess = WCProcessProtocol("accessing protocols through Twisted is fun!\n")
reactor.spawnProcess(wcProcess, 'wc', ['wc'])
reactor.run()
These are the methods that you can usefully override in your subclass of
ProcessProtocol :
.connectionMade() : This is called when the program is
started, and makes a good place to write data into the stdin pipe (using
self.transport.write ).
.outReceived(data) : This is called with data that was
received from the process’ stdout pipe. Pipes tend to provide data in
larger chunks than sockets (one kilobyte is a common buffer size), so you
may not experience the “random dribs and drabs” behavior typical of
network sockets, but regardless you should be prepared to deal if you
don’t get all your data in a single call. To do it properly,
outReceived ought to simply accumulate the data and put off
doing anything with it until the process has finished.
.errReceived(data) : This is called with data from the
process’ stderr pipe. It behaves just like outReceived .
.inConnectionLost : This is called when the reactor notices
that the process’ stdin pipe has closed. Programs don’t typically close
their own stdin, so this will probably get called when your
ProcessProtocol has shut down the write side with self.transport.loseConnection .
.outConnectionLost : This is called when the program closes
its stdout pipe. This usually happens when the program terminates.
.errConnectionLost : Same as
outConnectionLost , but for stderr instead of stdout.
.processExited(status) : This is called when the child
process has been reaped, and receives information about the process’ exit
status. The status is passed in the form of a Failure instance, created with a
.value that either holds a ProcessDone object if the process
terminated normally (it died of natural causes instead of receiving a
signal, and if the exit code was 0), or a ProcessTerminated object (with an
.exitCode attribute) if something went wrong.
.processEnded(status) : This is called when all the file
descriptors associated with the child process have been closed and the
process has been reaped. This means it is the last callback which will be
made onto a ProcessProtocol . The status parameter
has the same meaning as it does for processExited .
The base-class definitions of most of these functions are no-ops. This will result in all stdout and stderr being thrown away. Note that it is important for data you don’t care about to be thrown away: if the pipe were not read, the child process would eventually block as it tried to write to a full pipe.
The following are the basic ways to control the child process:
self.transport.write(data) : Stuff some data in the stdin
pipe. Note that this write method will queue any data that can’t
be written immediately. Writing will resume in the future when the pipe
becomes writable again.
self.transport.closeStdin : Close the stdin pipe. Programs
which act as filters (reading from stdin, modifying the data, writing to
stdout) usually take this as a sign that they should finish their job and
terminate. For these programs, it is important to close stdin when you’re
done with it, otherwise the child process will never quit.
self.transport.closeStdout : Not usually called, since you’re
putting the process into a state where any attempt to write to stdout will
cause a SIGPIPE error. This isn’t a nice thing to do to the poor
process.
self.transport.closeStderr : Not usually called, same reason
as closeStdout .
self.transport.loseConnection : Close all three pipes.
self.transport.signalProcess('KILL') : Kill the child
process. This will eventually result in processEnded being
called.
Here is an example that is rather verbose about exactly when all the
methods are called. It writes a number of lines into the wc
program and then parses the output.
#!/usr/bin/env python
# Copyright (c) Twisted Matrix Laboratories.
# See LICENSE for details.
from twisted.internet import protocol
from twisted.internet import reactor
import re
class MyPP(protocol.ProcessProtocol):
def __init__(self, verses):
self.verses = verses
self.data = ""
def connectionMade(self):
print "connectionMade!"
for i in range(self.verses):
self.transport.write("Aleph-null bottles of beer on the wall,\n" +
"Aleph-null bottles of beer,\n" +
"Take one down and pass it around,\n" +
"Aleph-null bottles of beer on the wall.\n")
self.transport.closeStdin() # tell them we're done
def outReceived(self, data):
print "outReceived! with %d bytes!" % len(data)
self.data = self.data + data
def errReceived(self, data):
print "errReceived! with %d bytes!" % len(data)
def inConnectionLost(self):
print "inConnectionLost! stdin is closed! (we probably did it)"
def outConnectionLost(self):
print "outConnectionLost! The child closed their stdout!"
# now is the time to examine what they wrote
#print "I saw them write:", self.data
(dummy, lines, words, chars, file) = re.split(r'\s+', self.data)
print "I saw %s lines" % lines
def errConnectionLost(self):
print "errConnectionLost! The child closed their stderr."
def processExited(self, reason):
print "processExited, status %d" % (reason.value.exitCode,)
def processEnded(self, reason):
print "processEnded, status %d" % (reason.value.exitCode,)
print "quitting"
reactor.stop()
pp = MyPP(10)
reactor.spawnProcess(pp, "wc", ["wc"], {})
reactor.run()
The exact output of this program depends upon the relative timing of some un-synchronized events. In particular, the program may observe the child process close its stderr pipe before or after it reads data from the stdout pipe. One possible transcript would look like this:
% ./process.py
connectionMade!
inConnectionLost! stdin is closed! (we probably did it)
errConnectionLost! The child closed their stderr.
outReceived! with 24 bytes!
outConnectionLost! The child closed their stdout!
I saw 40 lines
processEnded, status 0
quitting
Main loop terminated.
%
Frequently, one just needs a simple way to get all the output from a
program. In the blocking world, you might use commands.getoutput from the standard library, but
using that in an event-driven program will cause everything else to stall
until the command finishes. (in addition, the SIGCHLD handler used by that
function does not play well with Twisted’s own signal handling). For these
cases, the twisted.internet.utils.getProcessOutput
function can be used. Here is a simple example:
from twisted.internet import protocol, utils, reactor
from twisted.python import failure
from cStringIO import StringIO
class FortuneQuoter(protocol.Protocol):
fortune = '/usr/games/fortune'
def connectionMade(self):
output = utils.getProcessOutput(self.fortune)
output.addCallbacks(self.writeResponse, self.noResponse)
def writeResponse(self, resp):
self.transport.write(resp)
self.transport.loseConnection()
def noResponse(self, err):
self.transport.loseConnection()
if __name__ == '__main__':
f = protocol.Factory()
f.protocol = FortuneQuoter
reactor.listenTCP(10999, f)
reactor.run()
If you only need the final exit code (like commands.getstatusoutput(cmd)[0] ), the twisted.internet.utils.getProcessValue function is
useful. Here is an example:
from twisted.internet import utils, reactor
def printTrueValue(val):
print "/bin/true exits with rc=%d" % val
output = utils.getProcessValue('/bin/false')
output.addCallback(printFalseValue)
def printFalseValue(val):
print "/bin/false exits with rc=%d" % val
reactor.stop()
output = utils.getProcessValue('/bin/true')
output.addCallback(printTrueValue)
reactor.run()
“stdin” , “stdout” , and “stderr” are just conventions. Programs which operate as filters generally accept input on fd0, write their output on fd1, and emit error messages on fd2. This is common enough that the standard C library provides macros like “stdin” to mean fd0, and shells interpret the pipe character “|” to mean “redirect fd1 from one command into fd0 of the next command” .
But these are just conventions, and programs are free to use additional file descriptors or even ignore the standard three entirely. The”childFDs” argument allows you to specify exactly what kind of files descriptors the child process should be given.
Each child FD can be put into one of three states:
Mapped to a parent FD: this causes the child’s reads and writes to come from or go to the same source/destination as the parent.
Feeding into a pipe which can be read by the parent.
Feeding from a pipe which the parent writes into.
Mapping the child FDs to the parent’s is very commonly used to send the child’s stderr output to the same place as the parent’s. When you run a program from the shell, it will typically leave fds 0, 1, and 2 mapped to the shell’s 0, 1, and 2, allowing you to see the child program’s output on the same terminal you used to launch the child. Likewise, inetd will typically map both stdin and stdout to the network socket, and may map stderr to the same socket or to some kind of logging mechanism. This allows the child program to be implemented with no knowledge of the network: it merely speaks its protocol by doing reads on fd0 and writes on fd1.
Feeding into a parent’s read pipe is used to gather output from the child, and is by far the most common way of interacting with child processes.
Feeding from a parent’s write pipe allows the parent to control the child. Programs like “bc” or “ftp” can be controlled this way, by writing commands into their stdin stream.
The “childFDs” dictionary maps file descriptor numbers (as will be seen by the child process) to one of these three states. To map the fd to one of the parent’s fds, simply provide the fd number as the value. To map it to a read pipe, use the string “r” as the value. To map it to a write pipe, use the string “w” .
For example, the default mapping sets up the standard stdin/stdout/stderr pipes. It is implemented with the following dictionary:
childFDs = { 0: "w", 1: "r", 2: "r" }
To launch a process which reads and writes to the same places that the parent python program does, use this:
childFDs = { 0: 0, 1: 1, 2: 2}
To write into an additional fd (say it is fd number 4), use this:
childFDs = { 0: "w", 1: "r", 2: "r" , 4: "w"}
When you provide a “childFDs” dictionary with more than the normal
three fds, you need additional methods to access those pipes. These methods
are more generalized than the .outReceived ones described above.
In fact, those methods (outReceived and
errReceived ) are actually just wrappers left in for
compatibility with older code, written before this generalized fd mapping was
implemented. The new list of things that can happen to your ProcessProtocol
is as follows:
.connectionMade : This is called when the program is
started.
.childDataReceived(childFD, data) : This is called with
data that was received from one of the process’ output pipes (i.e. where
the childFDs value was “r” . The actual file number (from the point of
view of the child process) is in “childFD” . For compatibility, the
default implementation of .childDataReceived dispatches to
.outReceived or .errReceived when “childFD”
is 1 or 2.
.childConnectionLost(childFD) : This is called when the
reactor notices that one of the process’ pipes has been closed. This
either means you have just closed down the parent’s end of the pipe (with
.transport.closeChildFD ), the child closed the pipe
explicitly (sometimes to indicate EOF), or the child process has
terminated and the kernel has closed all of its pipes. The “childFD”
argument tells you which pipe was closed. Note that you can only find out
about file descriptors which were mapped to pipes: when they are mapped to
existing fds the parent has no way to notice when they’ve been closed. For
compatibility, the default implementation dispatches to
.inConnectionLost , .outConnectionLost , or
.errConnectionLost .
.processEnded(status) : This is called when the child
process has been reaped, and all pipes have been closed. This insures that
all data written by the child prior to its death will be received before
.processEnded is invoked.
In addition to those methods, there are other methods available to influence the child process:
self.transport.writeToChild(childFD, data) : Stuff some
data into an input pipe. .write simply writes to
childFD=0.
self.transport.closeChildFD(childFD) : Close one of the
child’s pipes. Closing an input pipe is a common way to indicate EOF to
the child process. Closing an output pipe is neither very friendly nor
very useful.
GnuPG, the encryption program, can use additional file descriptors to accept a passphrase and emit status output. These are distinct from stdin (used to accept the crypttext), stdout (used to emit the plaintext), and stderr (used to emit human-readable status/warning messages). The passphrase FD reads until the pipe is closed and uses the resulting string to unlock the secret key that performs the actual decryption. The status FD emits machine-parseable status messages to indicate the validity of the signature, which key the message was encrypted to, etc.
gpg accepts command-line arguments to specify what these fds are, and then assumes that they have been opened by the parent before the gpg process is started. It simply performs reads and writes to these fd numbers.
To invoke gpg in decryption/verification mode, you would do something like the following:
class GPGProtocol(ProcessProtocol):
def __init__(self, crypttext):
self.crypttext = crypttext
self.plaintext = ""
self.status = ""
def connectionMade(self):
self.transport.writeToChild(3, self.passphrase)
self.transport.closeChildFD(3)
self.transport.writeToChild(0, self.crypttext)
self.transport.closeChildFD(0)
def childDataReceived(self, childFD, data):
if childFD == 1: self.plaintext += data
if childFD == 4: self.status += data
def processEnded(self, status):
rc = status.value.exitCode
if rc == 0:
self.deferred.callback(self)
else:
self.deferred.errback(rc)
def decrypt(crypttext):
gp = GPGProtocol(crypttext)
gp.deferred = Deferred()
cmd = ["gpg", "--decrypt", "--passphrase-fd", "3", "--status-fd", "4",
"--batch"]
p = reactor.spawnProcess(gp, cmd[0], cmd, env=None,
childFDs={0:"w", 1:"r", 2:2, 3:"w", 4:"r"})
return gp.deferred
In this example, the status output could be parsed after the fact. It could, of course, be parsed on the fly, as it is a simple line-oriented protocol. Methods from LineReceiver could be mixed in to make this parsing more convenient.
The stderr mapping (“2:2” ) used will cause any GPG errors to be emitted by the parent program, just as if those errors had caused in the parent itself. This is sometimes desirable (it roughly corresponds to letting exceptions propagate upwards), especially if you do not expect to encounter errors in the child process and want them to be more visible to the end user. The alternative is to map stderr to a read-pipe and handle any such output from within the ProcessProtocol (roughly corresponding to catching the exception locally).