Using Pexpect

Launch Multiple Subprocesses and Parse Output with Regex Matching

Pexpect is a powerful Python module that allows for controlling and automating interactive command-line programs. With its ability to spawn and communicate with subprocesses, and automate command-line input and output, it can be an incredibly useful tool for managing multiple processes and their output.

In this article, we'll explore how to use Pexpect to launch multiple subprocesses and process their standard input/output/error with regex matching. Specifically, we'll look at how to match lines that contain "Current frame rate=[30] for stream id [abcd]", where "30" and "abcd" are numeric and alphanumeric regex patterns, respectively.

First, let's review the basic syntax for launching a subprocess with Pexpect. Here's an example of how to launch a simple command:

import pexpect

process = pexpect.spawn('ls -la')
process.expect(pexpect.EOF)
print(process.before.decode())

In this example, we're using Pexpect to spawn the 'ls -la' command and then wait for the subprocess to finish before printing the output. The expect() method waits for the process to return an end-of-file (EOF) signal before returning, and the before attribute contains the output generated by the subprocess.

Now, let's take a look at how to use Pexpect to match lines that contain a specific pattern in the output of a subprocess. We can accomplish this by using the expect() method with a regular expression pattern. Here's an example:

import re
import pexpect

process = pexpect.spawn('some_command')
while True:
    index = process.expect([pexpect.TIMEOUT, re.compile(r'Current frame rate=\[(\d+)\] for stream id \[([a-zA-Z0-9]+)\]')])
    if index == 0:
        break
    framerate = process.match.group(1)
    stream_id = process.match.group(2)
    print(f"Current frame rate: {framerate} for stream id: {stream_id}")

In this example, we're using Pexpect to spawn the 'some_command' command and then repeatedly reading the subprocess output until we find a line that matches the regex pattern. The regex pattern matches lines that contain "Current frame rate=[30] for stream id [abcd]", where "30" and "abcd" are numeric and alphanumeric regex patterns, respectively.

The expect() method returns the index of the matching pattern, and the match attribute contains the matching object with the regex groups. We can extract the matched values for the frame rate and stream ID by using the group() method.

By combining these two examples, we can launch multiple subprocesses and process their output with regex matching. Here's an example:

import re
import pexpect

commands = ['command1', 'command2', 'command3']
processes = []

for command in commands:
    process = pexpect.spawn(command)
    processes.append(process)

for process in processes:
    while True:
        index = process.expect([pexpect.TIMEOUT, re.compile(r'Current frame rate=\[(\d+)\] for stream id \[([a-zA-Z0-9]+)\]')])
        if index == 0:
            break
        framerate = process.match.group(1)
        stream_id = process.match.group(2)
        print(f"Current frame rate: {framerate} for stream id: {stream_id}")

In this example, we're launching multiple subprocesses by iterating over a list of commands and adding each process to a list. Then, we iterate over the list of processes and use Pexpect to process their output. The expect() method waits for each process to output a line that matches the regex pattern before moving on to the next process.

By using Pexpect to match lines with a specific pattern in the output of multiple subprocesses, we can efficiently parse and process data from multiple sources in a single script. This approach can be particularly useful for data processing, automation, and monitoring applications.

However, it's worth noting that Pexpect can be resource-intensive, and it may not be the most efficient or performant solution for all use cases. In some cases, other Python libraries, such as subprocess, os, or threading, may be more appropriate.

In conclusion, Pexpect is a versatile tool that can be used for controlling and automating interactive command-line programs in Python. By using Pexpect to match lines with a specific pattern in the output of multiple subprocesses, we can efficiently parse and process data from multiple sources in a single script. However, it's important to carefully consider the resource requirements and performance characteristics of Pexpect when designing and implementing your solution.