Python Subprocess Multiple Stdin.write And Stdout.read

Thanks for taking time to answer the question. I am playing around with Python 3.4 and I have two simple python programs. One, a program called test.py that takes a user input and

Solution 1:

In general, you should use pexpect for interactive programs (dialog-based interactions).

Your specific issue might be caused by a python version mismatch (you think your code is executed using Python 3 while actually it might be executed using Python 2). The second issue (EOFError) is expected: either catch it in the child script or provide a signal for the child to exit (I use an empty line for that in the code example below).

Here's a Python 3 code that fails loudly on Python 2:

#!/usr/bin/env python3import sys
from subprocess import Popen, PIPE

with Popen([sys.executable, '-u', 'test.py'], stdin=PIPE, stdout=PIPE,
           universal_newlines=True, bufsize=1) as cat:
    for input_string in ["hello, world!", "and another line", ""]:
        print(input_string, file=cat.stdin, flush=True)
        print(cat.stdout.readline(), end='')

Note:

• sys.exectable is the current python executable (Python 3 in this case)
• universal_newlines=True enables the text mode (otherwise, cat.stdin and cat.stdout work with bytes, not strings)
• -u makes the child's output line-buffered (otherwise, you won't see anything until the child flushes its internal stdout buffer)
• the with-statement closes the pipes and waits for the child process to exit.

And here's the corresponding test.py:

#!/usr/bin/env python3import time

whileTrue:
    x = input("enter something...")
    ifnot x: # exit if the input is emptybreakprint(x)
    time.sleep(1)

Output

enter something...hello, world!
enter something...and another line
enter something...

Note: there is no new line after "enter something..."

It works but it is fragile, read Q: Why not just use a pipe (popen())? and use pexpect instead.

---

If the input is finite and it doesn't depend on the output then you could pass it all at once:

#!/usr/bin/env python3import sys
from subprocess import check_output

output = check_output([sys.executable, 'test.py'],
                      input="\n".join(["hello, world!", "and another line"]),
                      universal_newlines=True)
print(output, end='')

This version requires that the child handles EOF properly:

#!/usr/bin/env python3import time

whileTrue:
    try:
        x = input("enter something...")
    except EOFError:
        break# no more inputprint(x)
    time.sleep(1)

The output is the same (as shown above).