Question

I’m posing this question mostly out of curiosity. I’ve written some code that is doing some very time intensive work. So, before executing my workhorse function, I wrapped it up in a couple of calls to time.clock(). It looks something like this:

t1 = time.clock()
print this_function_takes_forever(how_long_parameter = 20)
t2 = time.clock()
print t2 - t1

This worked fine. My function returned correctly and t2 - t1 gave me a result of 972.29, or about 16 minutes.

However, when I changed my code to this

t1 = time.clock()
print this_function_takes_forever(how_long_parameter = 80)
t2 = time.clock()
print t2 - t1

My function still returned fine, but the result of t2 - t1 was:

None
-1741

I’m curious as to what implementation detail causes this. Both the None, and the negative number are perplexing to me. Does it have something to do with a signed type? How does this explain the None?

Answer 1

The Python docs say:

On Unix, return the current processor time as a floating point number expressed in seconds. The precision, and in fact the very definition of the meaning of “processor time”, depends on that of the C function of the same name

The manpage of the referenced C function then explains the issue:

Note that the time can wrap around. On a 32-bit system where CLOCKS_PER_SEC equals 1000000 this function will
return the same value approximately every 72 minutes.