Reliable Non Blocking Reads From Subprocess Stdout

Note: I have a process that writes one line to stdout ('print('hello')) and waits for raw_input. I run this process with p = subprocess.Popen, then call p.stdout.readline()....thi

Solution 1:

readline reads a single line from the file-like object PIPE, to read it all of it, simply wrap it in a while loop. You should also call sleep after each read to save on CPU cycles.

Here is a simple example:

import subprocess

p = subprocess.Popen(
    ['ls', '-lat'],
    shell=False,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
    stdin=subprocess.PIPE
)
while True:
    line = p.stdout.readline()
    if line == '':
        breakprint(line.strip())  # remove extra ws between lines

EDIT:

woah, sorry, I completely missed the part you were trying to read input in that other process...

So, fn your other process, looks something like:

print('Hello')
in = raw_input()

Then the print actually sends the content to the file-like object PIPE you passed earlier which has it's own buffering mechanism. This behavior is explained in the print() function docs

To solve this simply add a sys.stdout.flush() between your print and raw_input:

print('Hello')
sys.stdout.flush()  # "flush" the output to our PIPE
in = raw_input()