thread stack traces make it hard to see CPU bound problems. Run the script with no args, and read the bottom comment in each stack trace (there are three threads so you'll see three stack traces). Note that despite taking almost 100% CPU, the stack traces show the worker thread sleeping.

misbehave.py

import datetime
import time
import sys
import threading
import traceback
import subprocess
import random


def clear_screen():
    sys.stdout.write(chr(27) + "[2J")
    sys.stdout.flush()


def _dump_thread_stacks():
    clear_screen()
    sys.stdout.write("\n*** STACKTRACE - START %s ***\n\n" % datetime.datetime.now())
    code = []
    for thread_id, stack in sys._current_frames().items():
        code.append("\n# ThreadID: %s" % thread_id)
        for filename, lineno, name, line in traceback.extract_stack(stack):  # THIS IS THE MONITORING THREAD
            code.append('File: "%s", line %d, in %s' % (filename,
                                                        lineno, name))
            if line:
                code.append("  %s" % (line.strip()))
    for line in code:
        sys.stdout.write(line)
        sys.stdout.write("\n")
    sys.stdout.write("\n*** STACKTRACE - END ***\n\n")


def monitor():
    while True:
        time.sleep(1)
        _dump_thread_stacks()


def misbehave():
    while True:
        # Release the gil, so this is all you'll see.
        time.sleep(0.0001)   # THIS IS WHAT YOU'LL PROBABLY SEE IN THE THREAD STACK TRACE

        # Thousands of lines later, many function
        # calls away...
        _waste_cpu() # THIS IS WHAT YOU'D LIKE TO SEE


def _waste_cpu():
    for i in xrange(50000):
        'the' + 'real' + 'problem' + 'here'


def start():
    bad_thread = threading.Thread(target=misbehave)
    bad_thread.daemon = True
    monitor_thread = threading.Thread(target=monitor)
    monitor_thread.daemon = True
    bad_thread.start()
    monitor_thread.start()
    while True:
        time.sleep(1) # THIS IS THE MAIN THREAD


start()