I want to use python3 to parse the output of "command>systeminfo" into json

import json, subprocess

def get_systeminfo():
    # Command to run.
    command = ['systeminfo']

    # Run the commands and get the stdout.
    with subprocess.Popen(command, universal_newlines=True, stdout=subprocess.PIPE) as p:
        stdout, _ = p.communicate()

    return stdout


def dic_from_systeminfo(stdout):
    # Dic to store all info.
    dic = {}

    # Previous key name saved to reuse for dic values.
    prevkey = ''

    # Loop through each line of stdout and split by colon.
    for line in stdout.splitlines():
        key, sep, value = line.partition(':')

        if sep == ':':
            value = value.strip()

            if not key.startswith(' '):
                # Assign these keys with values of type dic.
                if key in ('Original Install Date', 'System Boot Time'):
                    value = {k: v.strip() for k, v in zip(('Date', 'Time'), value.split(','))}
                elif key in ('Processor(s)', 'Hotfix(s)', 'Network Card(s)'):
                    value = dict()

                # Add to dic and save key name.
                dic[key] = value
                prevkey = key
            else:
                # Remove [] characters and then add key and value to the value type dic.
                key = key.strip().replace('[', '').replace(']', '')

                if prevkey and isinstance(dic[prevkey], dict):
                    dic[prevkey][key] = value
                else:
                    print('Warning:', 'dic[' + prevkey + '] is not a dict value.')

    return dic

# Run Systeminfo and get stdout.
stdout = get_systeminfo()

# Get dic from stdout (or read file content).
dic = dic_from_systeminfo(stdout)

# Write the dic to file.
if dic:
    with open('systeminfo.json', 'w', encoding='utf-8') as w:
        json.dump(dic, w, indent=4)

The script executes the command systeminfo and gets the stdout.

The stdout is processed line by line in the for loop and then is split into key and value by using the str.partition method.

If the key name does not start with a space, then it is a root key. If it matches one of the special key names, it will be set with a value of a dictionary type, else it will be set with the current value. The key name is saved to prevkey so that if the else condition is triggered, prevkey can be used as the root key and the key and value will be set to that root key.

The square brackets are removed from the keys in the else condition, though that can be considered an optional preference.

If the dic is something, then it will be written to systeminfo.json.

If you only want certain keys then you can save the keys of interest to a separate dictionary and write that to file.

As the main code for reading from an existing file, use:

stdout = ''

# Read from existing file.
with open('sysinfo.txt') as r:
    stdout = r.read()

# Get dic from stdout (or read file content).
dic = dic_from_systeminfo(stdout)

# Write the dic to file.
if dic:
    with open('systeminfo.json', 'w', encoding='utf-8') as w:
        json.dump(dic, w, indent=4)