Check drift for high-resolution system timer

#!/usr/bin/env python

from time import time as getTickTime, sleep as tickSleep
from ctypes import byref, c_int64, windll
from matplotlib import pyplot as plt
import numpy as np

# The number of samples to get
N = 1000
# The sleep period between samples (in sec.)
sleep = .1

def getQPCTime():

    """
    Uses the Windows QueryPerformanceFrequency API to get the system time. This
    implements the high-resolution timer as used for example by PsychoPy and
    PsychoPhysics toolbox.

    Returns:
    A timestamp (float)
    """

    _winQPC(byref(_fcounter))
    return  _fcounter.value/_qpfreq

# Complicated ctypes magic to initialize the Windows QueryPerformanceFrequency
# API. Adapted from the psychopy.core.clock source code.
_fcounter = c_int64()
_qpfreq = c_int64()
windll.Kernel32.QueryPerformanceFrequency(byref(_qpfreq))
_qpfreq = float(_qpfreq.value)
_winQPC = windll.Kernel32.QueryPerformanceCounter
# Create empty numpy arrays to store the results
aQPC = np.empty(N, dtype=float)
aTick = np.empty(N, dtype=float)
aDrift = np.empty(N, dtype=float)
# Wait for a minute to allow the Python interpreter to settle down.
tickSleep(1)
# Get the onset timestamps for the timers.
onsetQPCTime, onsetTickTime = getQPCTime(), getTickTime()
# Repeatedly check both timers
print "QPC\ttick\tdrift"
for i in range(N):
    # Get the QPC time and the tickTime
    QPCTime = getQPCTime()
    tickTime = getTickTime()
    # Subtract the onset time
    QPCTime -= onsetQPCTime
    tickTime -= onsetTickTime
    # Determine the drift, such that > 1 is a relatively slowed QPC timer.
    drift = tickTime / QPCTime
    # Sleep to avoid too many samples.
    tickSleep(sleep)
    # Print output
    print "%.4f\t%.4f\t%.4f" % (QPCTime, tickTime, drift)
    # Save the results in the arrays
    aQPC[i] = QPCTime
    aTick[i] = tickTime
    aDrift[i] = drift

# The first drift sample should be discarded
aDrift = aDrift[1:]
# Create a nice plot of the results
plt.figure(figsize=(6.4, 3.2))
plt.rc('font', size=10)
plt.subplots_adjust(wspace=.4, bottom=.2)
plt.subplot(121)
plt.plot(aQPC, color='#f57900', label='QPC timer')
plt.plot(aTick, color='#3465a4', label='Tick timer')
plt.xlabel('Sample')
plt.ylabel('Timestamp (sec)')
plt.legend(loc='upper left')
plt.subplot(122)
plt.plot(aDrift, color='#3465a4', label='Timer drift')
plt.axhline(1, linestyle='--', color='black')
plt.xlabel('Sample')
plt.ylabel('tick / QPC')
plt.savefig('systemTimerDrift.png')
plt.savefig('systemTimerDrift.svg')
plt.show()