I noticed a colleague at work today checked in a change to his code to
switch back from subprocess.Popen to os.popen.  I asked him about it and he
reported that subprocess.Popen was about 10x slower than os.popen.  I asked
him for a simple test case, which is attached.  Here are my results with
Python 2.4 through 2.7 (aka CVS HEAD):

    tmp% python2.4 popentest.py 
    time with os.popen :  0.09
    time with subprocess.Popen :  2.27
    tmp% python2.5 popentest.py 
    time with os.popen :  0.03
    time with subprocess.Popen :  1.52
    tmp% python2.6 popentest.py 
    time with os.popen :  0.038824
    time with subprocess.Popen :  1.517056
    tmp% python2.7 popentest.py 
    time with os.popen :  0.033746
    time with subprocess.Popen :  1.512331

These times are on my Mac laptop, all writing to the local disk.  It seems
there was a bit of improvement in the 2.5 release but that it is still
miserably slow when compared with os.popen.

His original test used time.clock() as the timer.  I changed to time.time()
but got essentially the same result.

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