I'd like to start a new process using the subprocess module. I'd like the stdin to be a stream that never sends anything but also doesn't terminate. Basically the same kind of input stream that a program would get if I were to launch it in the shell and not type anything, ever. Is that possible?
Pass stdin=subprocess.PIPE to the Popen constructor, and then avoid doing anything -- like calling communicate() -- which would later close that FIFO.
Related
I have been trying to troubleshoot subprocess.PIPE with subprocesses with no luck.
I'm trying to pass commands to an always running process and receive the results without having to close/open the process each time.
Here is the main launching code:
launcher.py:
import subprocess
import time
command = ['python', 'listener.py']
process = subprocess.Popen(
command, bufsize=0,
stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT
)
# simulates sending a new command every 10 seconds
for x in range(1,10):
process.stdin.write(b'print\r\n')
process.stdin.flush()
time.sleep(10)
listener.py:
import sys
file = open('log.txt', 'w+')
while True:
file.write(sys.stdin.read(1))
file.close()
This is simplified to show relevent pieces. In the end I'll have threads listening on the stdout and stderr but for now I'm trying to troubleshoot the basics.
What I expect to happen: for each loop in launcher.py, the file.write() in listener.py would write.
What happens instead: everything writes when the loop closes and the program terminates, or I SIGTERM / CTRL-C the script.
I'm running this in Windows 8 Python 3.4.
It's almost as if stdin buffers until the process closes and then it passes through. I have buffsize=0 set, and I'm flushing, so that doesn't make sense to me. I thought either one or the other would be sufficient.
The subprocess is running in a different process, so the sleep in launcher should have no impact on the subprocess.
Does anyone have any ideas why this is blocking?
Update 1:
The same behaviour is also seen with the following code run from the console (python.exe stdinreader.py)
That is, when you type into the console while the program is running, nothing is written to the file.
stdinreader.py:
import sys
import os
file = open('log.txt', 'w+b')
while True:
file.write(sys.stdin.read(1))
file.close()
Adding a file.flush() just before file.write() solves this, but that doesn't help me with the subprocess because I don't have control of how subprocess flushes (which would be my return subprocess.PIPE). Maybe if I reinitialize that PIPE with open('wb') it will not buffer. I will try.
Update 2:
I seem to have isolated this problem to the subprocess being called which is not flushing after it's writes to stdout.
Is there anything I can do to force a flush on the stdout PIPE between parent and child without modifying the subprocess? The subprocess is magick.exe (imagemagick 7) running with args ['-script, '-']. From the point of view of the subprocess it has a stdout object of <_io.TextIOWrapper name='' mode='w' encoding='cp1252'>. I guess the subprocess will just open the default stdout objects on initialization and we can't really control whether it buffers or not.
The strange thing is that passing the child the normal sys.stdout object instead of subprocess.PIPE does not require the subprocess to .flush() after write.
Programs run differently depending on whether they are run from the console or through a pipe. If the console (a python process can check with os.stdin.isatty()), stdout data is line buffered and you see data promptly. If a pipe, stdout data is block buffered and you only see data when quite a bit has piled up or the program flushes the pipe.
When you want to grab program output, you have to use a pipe and the program runs in buffered mode. On linux, you can trick programs by creating a fake console (pseudo tty, pty, ...). The pty module, pexpect and others do that.
On windows, I don't know of any way to get it to work. If you control the program being run, have it flush often. Otherwise, glare futilely at the Windows logo. You can even mention the problem on your next blind date if you want it to end early. But I can't think of anything more.
(if somebody knows of a fix, I'd like to hear it. I've seen some code that tries to open a Windows console and screen scrape it, but those solutions keep losing data. It should work if there is a loopback char device out there somewhere).
The problem was that the subprocess being called was not flushing after writing to stdout. Thanks to J.F. and tdelaney for pointing me in the right direction. I have raised this with the developer here: http://www.imagemagick.org/discourse-server/viewtopic.php?f=2&t=26276&p=115545#p115545
There doesn't appear to be a work-around for this in Windows other than to alter the subprocess source. Perhaps if you redirected the output of the subprocess to a NamedTemporaryFile that might work, but I have not tested it and I think it would be locked in Windows so only one of the parent and child could open it at once. Not insurmountable but annoying. There might also be a way to exec the application through unixutils port of stdbuf or something similar as J.F. suggested here: Python C program subprocess hangs at "for line in iter"
If you have access to the source code of the subprocess you're calling you can always recompile it with buffering disabled. It's simple to disable buffering on stdout in C:
setbuf(stdout, NULL)
or set per-line buffering instead of block buffering:
setvbuf(stdout, (char *) NULL, _IOLBF, 0);
See also: Python C program subprocess hangs at "for line in iter"
Hope this helps someone else down the road.
can you try to close the pipe at the end of listener.py? i think that is the issue
I recently noted in Python the subprocess.Popen() has an argument:
stdout=None(default)
I also saw people using stdout=subprocess.PIPE.
What is the difference? Which one should I use?
Another question would be, why the wait() function can't wait until the process is really done sometimes? I used:
a = sp.Popen(....,shell=True)
a.wait()
a2 = sp.Popen(...,shell=True)
a2.wait()
sometimes the a2 command is executed before the command a is done.
stdout=None means, the stdout-handle from the process is directly inherited from the parent, in easier words it basically means, it gets printed to the console (same applies for stderr).
Then you have the option stderr=STDOUT, this redirects stderr to the stdout, which means the output of stdout and stderr are forwarded to the same file handle.
If you set stdout=PIPE, Python will redirect the data from the process to a new file handle, which can be accessed through p.stdout (p beeing a Popen object). You would use this to capture the output of the process, or for the case of stdin to send data (constantly) to stdin.
But mostly you want to use p.communicate, which allows you to send data to the process once (if you need to) and returns the complete stderr and stdout if the process is completed!
One more interesting fact, you can pass any file-object to stdin/stderr/stdout, e.g. also a file opened with open (the object has to provide a fileno() method).
To your wait problem. This should not be the case! As workaround you could use p.poll() to check if the process did exit! What is the return-value of the wait call?
Furthermore, you should avoid shell=True especially if you pass user-input as first argument, this could be used by a malicious user to exploit your program! Also it launches a shell process which means additional overhead. Of course there is the 1% of cases where you actually need shell=True, I can't judge this with your minimalistic example.
stdout=None means that subprocess prints to whatever place your script prints
stdout=PIPE means that subprocess' stdout is redirected to a pipe that you should read e.g., using process.communicate() to read all at once or using process.stdout object to read via a file/iterator interfaces
I am writing a test framework in Python for a command line application. The application will create directories, call other shell scripts in the current directory and will output on the Stdout.
I am trying to treat {Python-SubProcess, CommandLine} combo as equivalent to {Selenium, Browser}. The first component plays something on the second and checks if the output is expected. I am facing the following problems
The Popen construct takes a command and returns back after that command is completed. What I want is a live handle to the process so I can run further commands + verifications and finally close the shell once done
I am okay with writing some infrastructure code for achieveing this since we have a lot of command line applications that need testing like this.
Here is a sample code that I am running
p = subprocess.Popen("/bin/bash", cwd = test_dir)
p.communicate(input = "hostname") --> I expect the hostname to be printed out
p.communicate(input = "time") --> I expect current time to be printed out
but the process hangs or may be I am doing something wrong. Also how do I "grab" the output of that sub process so I can assert that something exists?
subprocess.Popen allows you to continue execution after starting a process. The Popen objects expose wait(), poll() and many other methods to communicate with a child process when it is running. Isn't it what you need?
See Popen constructor and Popen objects description for details.
Here is a small example that runs Bash on Unix systems and executes a command:
from subprocess import Popen, PIPE
p = Popen (['/bin/sh'], stdout=PIPE, stderr=PIPE, stdin=PIPE)
sout, serr = p.communicate('ls\n')
print 'OUT:'
print sout
print 'ERR:'
print serr
UPD: communicate() waits for process termination. If you do not need that, you may use the appropriate pipes directly, though that usually gives you rather ugly code.
UPD2: You updated the question. Yes, you cannot call communicate twice for a single process. You may either give all commands you need to execute in a single call to communicate and check the whole output, or work with pipes (Popen.stdin, Popen.stdout, Popen.stderr). If possible, I strongly recommend the first solution (using communicate).
Otherwise you will have to put a command to input and wait for some time for desired output. What you need is non-blocking read to avoid hanging when there is nothing to read. Here is a recipe how to emulate a non-blocking mode on pipes using threads. The code is ugly and strangely complicated for such a trivial purpose, but that's how it's done.
Another option could be using p.stdout.fileno() for select.select() call, but that won't work on Windows (on Windows select operates only on objects originating from WinSock). You may consider it if you are not on Windows.
Instead of using plain subprocess you might find Python sh library very useful:
http://amoffat.github.com/sh/
Here is an example how to build in an asynchronous interaction loop with sh:
http://amoffat.github.com/sh/tutorials/2-interacting_with_processes.html
Another (old) library for solving this problem is pexpect:
http://www.noah.org/wiki/pexpect
Using python pty module, i want to send some commands to the terminal emulator, using a function as stdin (as pty module wants), and then force quitting. I thought about something like
import pty
cmnds = ['exit\n', 'ls -al\n']
# Command to send. I try exiting as last command, but it doesn't works.
def r(fd):
if cmnds:
cmnds.pop()
# It seems is not executing sent commands ('ls -al\n')
else:
# Can i quit here? Can i return EOF?
pass
pty.spawn('/bin/sh', r)
Thank you
Firstly, the pty module does not allow you to communicate with the terminal emulator Python is running in. Instead, it allows Python to pretend to be a terminal emulator.
Looking at the source-code of pty.spawn(), it looks like it is designed to let a spawned process take over Python's stdin and stdout while it runs, which is not what you want.
If you just want to spawn a shell, send commands to it, and read the output, you probably want Python's subprocess module (in particular, if there's just one command you want to run, the subprocess.Popen class' .communicate() method will be helpful).
If you really, really need the sub-process to be running in a pty instead of a pipe, you can use os.openpty() to allocate a master and a slave file descriptor. Use the slave file descriptor as the subprocess' stdin and stdout, then write your commands to the master file descriptor and read the responses back from it.
I'd like to execute multiple commands in a standalone application launched from a python script, using pipes. The only way I could reliably pass the commands to the stdin of the program was using Popen.communicate but it closes the program after the command gets executed. If I use Popen.stdin.write than the command executes only 1 time out of 5 or so, it does not work reliable. What am I doing wrong?
To elaborate a bit :
I have an application that listens to stdin for commands and executes them line by line.
I'd like to be able to run the application and pass various commands to it, based on the users interaction with a GUI.
This is a simple test example:
import os, string
from subprocess import Popen, PIPE
command = "anApplication"
process = Popen(command, shell=False, stderr=None, stdin=PIPE)
process.stdin.write("doSomething1\n")
process.stdin.flush()
process.stdin.write("doSomething2\n")
process.stdin.flush()
I'd expect to see the result of both commands but I don't get any response. (If I execute one of the Popen.write lines multiple times it occasionally works.)
And if I execute:
process.communicate("doSomething1")
it works perfectly but the application terminates.
If I understand your problem correctly, you want to interact (i.e. send commands and read the responses) with a console application.
If so, you may want to check an Expect-like library, like pexpect for Python: http://pexpect.sourceforge.net
It will make your life easier, because it will take care of synchronization, the problem that ddaa also describes. See also:
http://www.noah.org/wiki/Pexpect#Q:_Why_not_just_use_a_pipe_.28popen.28.29.29.3F
The real issue here is whether the application is buffering its output, and if it is whether there's anything you can do to stop it. Presumably when the user generates a command and clicks a button on your GUI you want to see the output from that command before you require the user to enter the next.
Unfortunately there's nothing you can do on the client side of subprocess.Popen to ensure that when you have passed the application a command the application is making sure that all output is flushed to the final destination. You can call flush() all you like, but if it doesn't do the same, and you can't make it, then you are doomed to looking for workarounds.
Your code in the question should work as is. If it doesn't then either your actual code is different (e.g., you might use stdout=PIPE that may change the child buffering behavior) or it might indicate a bug in the child application itself such as the read-ahead bug in Python 2 i.e., your input is sent correctly by the parent process but it is stuck in the child's internal input buffer.
The following works on my Ubuntu machine:
#!/usr/bin/env python
import time
from subprocess import Popen, PIPE
LINE_BUFFERED = 1
#NOTE: the first argument is a list
p = Popen(['cat'], bufsize=LINE_BUFFERED, stdin=PIPE,
universal_newlines=True)
with p.stdin:
for cmd in ["doSomething1\n", "doSomethingElse\n"]:
time.sleep(1) # a delay to see that the commands appear one by one
p.stdin.write(cmd)
p.stdin.flush() # use explicit flush() to workaround
# buffering bugs on some Python versions
rc = p.wait()
It sounds like your application is treating input from a pipe in a strange way. This means it won't get all of the commands you send until you close the pipe.
So the approach I would suggest is just to do this:
process.stdin.write("command1\n")
process.stdin.write("command2\n")
process.stdin.write("command3\n")
process.stdin.close()
It doesn't sound like your Python program is reading output from the application, so it shouldn't matter if you send the commands all at once like that.