Unable to read stdout from python with popen in lua

Asked Feb 13 '19 at 09:45

Active Feb 13 '19 at 09:45

Viewed 393 times

I am trying to read the stdout of a simple python script

    import time
    while True:
        print("test")
        time.sleep(1)

with the following lua code

    local p = assert(io.popen("/usr/bin/python test.py", "r"))
    if not p then
        print("error")
    end
    print("process: "..tostring(p))

    while p do
        local line = p:read("*l")
        if line then
            print("line: "..tostring(line))
        end
    end

This prints a process file identifier but no output.

Substituting yes as command shows that the lua code in itself should work. However no method of calling the python script (including shebang) will produce any output. Changing to sys.stdout.write("test" + "\n") doesn't make a difference either.

I tried it on two machines running archlinux and debian. The first one is running python 3.7.2 and lua 5.3.5.

asked Feb 13 '19 at 09:45

James Marlow

Could you remove the while loop in the `python` script and try ? – han solo Feb 13 '19 at 09:49
Ok, that does produce output once. – James Marlow Feb 13 '19 at 09:53
It also works in a for loop. Does this mean python buffers the prints until the while loop terminates? – James Marlow Feb 13 '19 at 10:09
2

Try, `sys.stdout.write("test" + "\n"); sys.stdout.flush()` :) – han solo Feb 13 '19 at 10:27
Cool, thanks! I was struggling for days with this thinking the problem was with love2d or lua. I also found [this answer](https://stackoverflow.com/a/36131331/11055460) that shows some other ways. – James Marlow Feb 13 '19 at 11:03

Unable to read stdout from python with popen in lua

0 Answers0