Catching string from stdout with Python

Using a Python “wrapper” script you can catch the output of an executing application, which is very useful when you want to perform certain tasks depending on what is being printed to stdout by the application.

This is a simplified example of how I do it:

# Imports
import os, sys, subprocess

# Build command
command = [ 'python', os.join.path('/path/to', 'scriptFile.py') ]

# Execute command
p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

# Read stdout and print each new line
sys.stdout.flush()
for line in iter(p.stdout.readline, b''):

    # Print line
    sys.stdout.flush()
    print(">>> " + line.rstrip())


    # Look for the string 'Render done' in the stdout output
    if 'Render done' in line.rstrip():

        # Write something to stdout
        sys.stdout.write('Nice job on completing the render, I am executing myFunction()\n' )
        sys.stdout.flush()

        # Execute something
        myFunction()

Catching stuff from stdout for Pixar's Tractor

Here’s an example of catching the output of a V-Ray for Maya render and outputting TR_PROGRESS nnn% to stdout whenever a percentage is being printed to stdout, which makes Pixar’s Tractor show a task progress in the task node tree.

# Imports
import os, sys, subprocess, re

# Build command
command = ['python', os.join.path('/path/to', 'mayaWrapper.py'), '-r', 'vray']

# Execute command
p = subprocess.Popen( command, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

# Read stdout and print each new line
sys.stdout.flush()
for line in iter(p.stdout.readline, b''):
    sys.stdout.flush()
    print(">>> " + line.rstrip())

    # Tractor Progress
    match = re.search('[\d]* %', line.rstrip() )            # Detect percentage without period
    if not match:
        match = re.search('[\d.\d]* %', line.rstrip() )     # Detect percentage with period
    if match:
        percent = match.group(0).replace('%','').replace(' ','')
        try:
            percent = float(percent)
            percent = int(percent)
            percent = str(percent)
        except:
            pass
        sys.stdout.write('TR_PROGRESS ' + str(percent) + '%\n' )
        sys.stdout.flush()

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