Using the timeit module

The most important thing you need to know about  Python  code optimization is to never write timing functions yourself.

Timing a very short code is complicated. How much time does the processor have to run this code? Is there something running in the background? Every modern computer runs continuous or intermittent programs in the background. Small oversights can ruin your 100-year plan, background services are occasionally " wake up " in the last thousandth of a second to do things like check for mail, connect to timed communication servers, check for app updates, scan for viruses, see if a disk is inserted into an optical drive meaningful things like that. Turn everything off and disconnect from the network before starting the timed test. Make sure everything is off again and turn off services that keep checking to see if the network is back up, etc.

Next is the variable factor introduced by the timing frame itself. Does the Python  interpreter cache method name lookups? Cache compiled results of code blocks? What about regular expressions? Does your code have side effects when it runs repeatedly? Don't forget that the results of your work will be presented in units smaller than seconds, and that small mistakes in your timing framework will distort the results irreparably.

 There is a saying in the Python community: " Python  carries its own battery. " Don't write your own timing framework. Python 2.3 has a  perfect timing tool  called  timeit .

Example 18.2.  Timeit  introduction

If you have not downloaded the sample programs that accompany this book, you can  download this and other sample programs .

>>> import timeit
>>> t = timeit.Timer("soundex.soundex('Pilgrim')",
...     "import soundex")   >>> t.timeit()              8.21683733547
>>> t.repeat(3, 2000000)    [16.48319309109, 16.46128984923, 16.44203948912]

	The timeit  module defines the  Timer  class that accepts two parameters. Both parameters are strings. The first parameter is the statement you want to time, here you are calling the Soundex function with the 'Pilgrim' parameter.  The second parameter passed to  Timer is the import statement that builds the environment for the first parameter statement. Internally, timeit  builds a separate virtual environment, manually executes the build statements (imports the  soundex  module), and then manually compiles and executes the timed statements (calls the Soundex function).
	As long as you have a  Timer  object, the easiest thing to do is to call  timeit() , which calls your function a million times and returns the elapsed seconds.
	The other main method of the Timer  object is  repeat() , which accepts two optional parameters. The first parameter is the number of times to repeat the entire test, and the second parameter is the number of times the timed statement is called in each test. Both parameters are optional and their default values ​​are  3  and  1000000 respectively . The repeat()  method returns a list of elapsed times for each test loop recorded in seconds.

	You can use the timeit module on the command line   to test an existing  Python  program without modifying the code.  See the documentation for command line options at  http://docs.python.org/lib/node396.html .

Note that  repeat()  returns a list of times. Due  to small changes in processor time used by Python  timers (or those nasty background processes you can't root out), it's almost impossible to have duplicates in these times. Your first thought might be to say, " Let's average to get the real data. "

In fact, that's almost certainly wrong. Changes to your code or the  Python  interpreter may shorten the time, and those nasty background processes that cannot be removed or other  factors other than the Python  interpreter may increase the time. If the difference between the timing results is more than a few percent, there are too many variables to trust the results, if not, take the minimum value and discard the other results.

Python  has a handy  min  function that returns the smallest value in the input list:

>>> min(t.repeat(3, 1000000))
8.22203948912

	The timeit  module is only used when you know which piece of code needs to be optimized. If you have a large  Python  program and don't know where your performance problems lie, check out the  hotshot  module .