Calling external program from within python

[Emmanouil Angelakis]

Hi,

I am tryiong to do something obviously trivial such as:
I have a c program called "tsys2list" that when it is ran it asks the user to give the value of "tcal" which is a variable. I want to call the "tsys2list" from within a pyrthon script lets call it "gamma.py" >>>but<<< I want to pass the value of "tcal" to th eprogram automatically and not by interacting with the keyboard.

How do I do that?

thanks i advance!
manolis

 

[Mike Driscoll]

Hi,

I am tryiong to do something obviously trivial such as:
I have a c program called "tsys2list" that when it is ran it asks the user to give the value of "tcal" which is a variable. I want to call the  "tsys2list" from within a pyrthon script lets call it "gamma.py" >>>but<<< I want to pass the value of "tcal" to th eprogram automatically and not by interacting with the keyboard.

How do I do that?

thanks i advance!
manolis

There are probably many ways to do this. I would recommend checking
out the subprocess module and see if it does what you want. Or you
could learn a little Tkinter or wxPython and use that to get the
user's variable. Or you could even do it via the command line using
the "raw_input" command.

http://docs.python.org/lib/module-subprocess.html

Mike

 

[Diez B. Roggisch]

There are probably many ways to do this. I would recommend checking
out the subprocess module and see if it does what you want.

This will only work if the program can be fully controlled by
commandline arguments. If interaction is required, the OP might consider
using pexpect.

Or you
could learn a little Tkinter or wxPython and use that to get the
user's variable. Or you could even do it via the command line using
the "raw_input" command.

I fail to see how *gathering* input (regardless of the method) solves
the problem of *passing* input to a subprocess.

Diez

 

[Diez B. Roggisch]

Why do you say that? You can interact with programs via
stdin/stdout using the subprocess module.

Because usually if a program *prompts* the user to enter input (and that
was what I read from the OP's post), one has to deal with pseudo
terminals, not with stdin/out.

Pexpect is a good option, but it's just an automation layer on
top of the same facilities that subprocess provides.

AFAIK it's more than that. I'm not an expert on pseudo terminals, but
AFAIK setting using module pty isn't possible using subprocess.

Diez

 

[oj]

Because usually if a program *prompts* the user to enter input (and that
was what I read from the OP's post), one has to deal with pseudo
terminals, not with stdin/out.

How does the program writing some text before taking input change how
it takes input?

If it runs in a terminal and takes input from the user via keyboard,
as in the user types a response and presses enter, it's most likely
using stdin, in which case subprocess would be perfect.

Perhaps that OP can clarify how the tcal program takes input?

 

[Diez B. Roggisch]

How does the program writing some text before taking input change how
it takes input?

If it runs in a terminal and takes input from the user via keyboard,
as in the user types a response and presses enter, it's most likely
using stdin, in which case subprocess would be perfect.

Try using SSH with subprocess and prepare to be disappointed when it
asks for a password.

See this for a discussion:

http://mail.python.org/pipermail/distutils-sig/2007-May/007553.html


Diez

 

[Marc 'BlackJack' Rintsch]

How does the program writing some text before taking input change how
it takes input?

It might not write the text before taking input because it is buffered.
But if the controlling program waits for some text before sending input to
the other program all hangs.

If it runs in a terminal and takes input from the user via keyboard,
as in the user types a response and presses enter, it's most likely
using stdin, in which case subprocess would be perfect.

But buffering messes with the order of inputs and outputs. Buffering is
different if you "talk" to a real (or pseudo) terminal or to a file or
pipe and the calling program can't influence the called programs buffering
except if it pretends to be a terminal, which pexpect does.

Ciao,
Marc 'BlackJack' Rintsch

 

[Mike Driscoll]

This will only work if the program can be fully controlled by
commandline arguments. If interaction is required, the OP might consider
using pexpect.


I fail to see how *gathering* input (regardless of the method) solves
the problem of *passing* input to a subprocess.

Diez

My understanding of the OP's request was that "tsys2list" was a custom
script of some sort that gathered info from the user, so I wanted to
know why he didn't just skip calling it and write something in Tkinter
or wxPython or even via the CLI instead.

I'm probably just not understanding something.

Mike

 

[Diez B. Roggisch]

My understanding of the OP's request was that "tsys2list" was a custom
script of some sort that gathered info from the user, so I wanted to
know why he didn't just skip calling it and write something in Tkinter
or wxPython or even via the CLI instead.

I'm probably just not understanding something.

The program is doing some work, and needs to be told what actually to
do. It does so by asking the user, sure. But then it *works*. I think
that's pretty clear from

"""
I have a c program called "tsys2list" that when it is ran it asks the
user to give the value of "tcal" which is a variable. I want to call the
"tsys2list" from within a pyrthon script lets call it "gamma.py" automatically and not by interacting with the keyboard.
"""

So replacing it with something that asks the same questions leaves us
with the work undone...


Diez

 

[Michael Tobis]

These answers are too elaborate and abstract for the question.

Emmanouil,

Here is a program "myprog" which takes input and writes output to a
file. It happens to be python but it could be anything.

#####
#!/usr/bin/env python
a = int(raw_input("enter thing 1 "))
b = int(raw_input("enter thing 2 "))
c = raw_input("enter output file name ")
f = open(c,"w")
f.write("answer is %s\n" % str(a + b))
f.close()
#####

Here is python code which runs that program

#####
import os

f = os.popen("/path/to/myprog","w")
f.write("3\n4\noutputname\n") #can also do three separate writes if
that is more convenient
f.close()
#####

For some reason os.popen is deprecated in favor of the more verbose
subprocess.Popen, but this will work for a while.

Here is an even simpler approach that will take you a long way. This
should be very easy to understand.

#####
import os

params = open("temp","w")
params.write("3\n")
params.write("4\n")
params.write("outputname2\n")
params.close()

os.system("/path/to/myprog < temp")
#####

If you want python to pick up the stdout of your program as well look
into popen2.

Try basing your solution on those and if you have any problems let us
know what they are. In most cases such as you describe these will
work.

mt

 

[Terry Reedy]

For some reason os.popen is deprecated in favor of the more verbose
subprocess.Popen, but this will work for a while.

As explained in
http://www.python.org/dev/peps/pep-0324/
subprocess consolidated replaced several modules and functions (popen*,
system, spawn*, ???)with better security, exception handling, and
flexibility. The deprecated modules are gone in 3.0, so the OP might
want to start with subprocess now.