如何在命令提示符下将 python 变量传递给进程

Question

I have a python loop that at each iteration is creating a new image in a new path. I want to send that path into a process previously executed that is waiting for a path at the command prompt.

In more detail:

I am running a self-driving simulator in python and every 5 frames I want to test that current frame (that is saved in a RAM disk) into an object detector algorithm I have trained (that spends arround 9ms to detect my object BUT 2 seconds to open the process). Actually, I execute the trained algorithm by using the subprocess module, but the problem I have is that when that process is opened (I just open the process once, when I run the main script) it is waiting for a path image. I believe that with your tips I am close to the answer but I don't face how to pass that path image to this subprocess that is waiting for it at each 5 frames iteration.

PD: I am on Windows, Python 3.5.4

Do you know what can I do?

Answer 1

If you're on a *nix environment, and I'm understanding what you want, pipes provides what you want to do quite elegantly. I don't know Windows, but maybe the same concept could be used there.

Here's a simple example that illustrates this:

Pipe1a.py:

#!/usr/bin/env python

import time
import sys

for i in range(10):
    time.sleep(1) # represent some processing delay
    print("filename{}.jpg".format(i))
    sys.stdout.flush()

Pipe1b.py:

#!/usr/bin/env python

import sys

while True:
    line = sys.stdin.readline()
    if len(line) == 0:
        break
    print "Processing image '{}'".format(line.strip())

If you made both of these scripts executable, then you could chain them together via a pipe at the command prompt:

> Pipe1a.py | Pipe1b.py

Resulting output:

Processing image 'filename0.jpg'
Processing image 'filename1.jpg'
Processing image 'filename2.jpg'
Processing image 'filename3.jpg'
Processing image 'filename4.jpg'
Processing image 'filename5.jpg'
Processing image 'filename6.jpg'
Processing image 'filename7.jpg'
Processing image 'filename8.jpg'
Processing image 'filename9.jpg'

The concept here is that one process writes data to its stdout and a second process reads that data from its stdin .

If you don't want to use the command prompt to string these together, but rather want to run a single script, you can do this same thing, with pretty much the same code, using the Python subprocess1 module. For this example, you could have the Pipe1b.py program run the Pipe1a.py program via subprocess.Popen and then process its output in this same way.

Answer 2

Thank you for all your comments and for your help. Finally I have achieved it.

I use the library 'pexpect' that allows to launch a program inside a python script with the function process = popen_spawn.PopenSpawn . Then, a function from that library called 'send' allows to pass an argument to that running process ( process.send(arg) ).

In my case, I launch the program (.exe) at the beginning of the script defining that instance as a global variable. Then I just have to execute the send function at each iteration.