Extract just one data point from powermetrics, suitable for graphing

powermetrics_reader.py

#!python -u

import subprocess
import plistlib
import sys

"""
Print selected power metrics from /usr/bin/powermetrics at intervals, suitable
for graphing.
"""

# available data is determined by which categories we ask for (the -s option)
interval = sys.argv[1] if len(sys.argv) > 1 else 1000
cmd = f"sudo powermetrics -i {interval} -s cpu_power --format plist"

# Execute the long-running command, which produces one plist to stdout every
# interval, spread across multiple lines.
# For each line of output from `powermetrics`:
#   - accumulate lines into a bytearray
#   When "</plist>" is seen:
#    - parse the accumulated bytearray into a plist
#    - print the combined power consumption
#    - clear the bytearray
pl = bytearray()
# using shell mode here because we need sudo for powermetrics
for line in subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE).stdout:
    pl.extend(line)
    # look for the plist close tag
    if "</plist>" in str(line):
        # powermetrics adds a null byte at the start of each plist after the
        # first, which breaks the plist parser.  Remove it.
        if pl[0] == 0:
            pl = pl[1:]
        d = plistlib.loads(pl[0:])
        print(d.get('processor').get('combined_power'))
        #print(d.get('all_tasks').get('cputime_ms_per_s'))  # requires "all_tasks" category
        pl = bytearray()