Python subprocess module - unexpected behavior

Question

I need to interface a C console program (as subprocess) with Python using stdin/stdout.

the C program is more o less it:

    tmp = 0.0;  
    printf("\ninput>>");
    scanf_s("%f",&tmp);
    printf ("\ninput was: %f",tmp);

    tmp = 0.0;
    printf("\ninput>>");
    scanf_s("%f",&tmp);
    printf ("\ninput was: %f",tmp);

    tmp = 0.0;
    printf("\ninput>>");
    scanf_s("%f",&tmp);
    printf ("\ninput was: %f",tmp);

Using python subprocess module I need to read data from this program, the write something, then read again and so on. I used the following code:

>>> p=subprocess.Popen(['C:\T.exe'],stdout=subprocess.PIPE,stdin=subprocess.PIPE)
>>> o,i=communicate('123\n')

the output of o is:

input>>
input was: 123.000000
input>>
input was: 0.000000
input>>
input was: 0.000000

I would expect the subprocess to wait on input until another o,i=communicate() call. Why it is proceding to the end of the program without any input? how to fix it?

This has bitten me once in the past as well. :)

Skurmedel
– Skurmedel

2010-06-28 09:12:59 +00:00
Commented Jun 28, 2010 at 9:12 — Skurmedel
– Skurmedel, Commented Jun 28, 2010 at 9:12

Philipp · Accepted Answer · 2010-06-28 09:11:45Z

3

There can be at most one call to communicate() for each process, because communicate() waits for the child process to terminate. To repeatedly read and write from/to a process's standard streams, use the stdout and stdin attributes of the Popen class.

answered Jun 28, 2010 at 9:11

Philipp

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4 Comments

Vladimir Keleshev Over a year ago

if i use p.stdout.read(10) then the python shell is not responding until I close the process manually. then it outputs ''. same with .readline and .readlines. What that could be?

Philipp Over a year ago

read(10) waits until it encounters EOF or has read 10 bytes. I have little experience with that, but you should use select or background threads to poll and read from the streams. There is also a warning about deadlocks in the subprocess documentation.

Vladimir Keleshev Over a year ago

what kind of "select" do you mean?

Vladimir Keleshev Over a year ago

btw, p.stdin.write() works perfect. and all p.stdout.read*() works fine only after termination of the subprocess.

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