26.6. "timeit" — Measure execution time of small code snippets
**************************************************************

2.3 版新加入.

**Source code:** Lib/timeit.py

======================================================================

This module provides a simple way to time small bits of Python code.
It has both a 命令執行列介面 as well as a callable one.  It avoids a
number of common traps for measuring execution times. See also Tim
Peters』 introduction to the 「Algorithms」 chapter in the *Python
Cookbook*, published by O’Reilly.


26.6.1. 基礎範例
================

The following example shows how the 命令執行列介面 can be used to
compare three different expressions:

   $ python -m timeit '"-".join(str(n) for n in range(100))'
   10000 loops, best of 3: 40.3 usec per loop
   $ python -m timeit '"-".join([str(n) for n in range(100)])'
   10000 loops, best of 3: 33.4 usec per loop
   $ python -m timeit '"-".join(map(str, range(100)))'
   10000 loops, best of 3: 25.2 usec per loop

This can be achieved from the Python Interface with:

   >>> import timeit
   >>> timeit.timeit('"-".join(str(n) for n in range(100))', number=10000)
   0.8187260627746582
   >>> timeit.timeit('"-".join([str(n) for n in range(100)])', number=10000)
   0.7288308143615723
   >>> timeit.timeit('"-".join(map(str, range(100)))', number=10000)
   0.5858950614929199

Note however that "timeit" will automatically determine the number of
repetitions only when the command-line interface is used.  In the
Examples section you can find more advanced examples.


26.6.2. Python Interface
========================

The module defines three convenience functions and a public class:

timeit.timeit(stmt='pass', setup='pass', timer=<default timer>, number=1000000)

   Create a "Timer" instance with the given statement, *setup* code
   and *timer* function and run its "timeit()" method with *number*
   executions.

   2.6 版新加入.

timeit.repeat(stmt='pass', setup='pass', timer=<default timer>, repeat=3, number=1000000)

   Create a "Timer" instance with the given statement, *setup* code
   and *timer* function and run its "repeat()" method with the given
   *repeat* count and *number* executions.

   2.6 版新加入.

timeit.default_timer()

   Define a default timer, in a platform-specific manner.  On Windows,
   "time.clock()" has microsecond granularity, but "time.time()"』s
   granularity is 1/60th of a second.  On Unix, "time.clock()" has
   1/100th of a second granularity, and "time.time()" is much more
   precise.  On either platform, "default_timer()" measures wall clock
   time, not the CPU time.  This means that other processes running on
   the same computer may interfere with the timing.

class timeit.Timer(stmt='pass', setup='pass', timer=<timer function>)

   Class for timing execution speed of small code snippets.

   The constructor takes a statement to be timed, an additional
   statement used for setup, and a timer function.  Both statements
   default to "'pass'"; the timer function is platform-dependent (see
   the module doc string). *stmt* and *setup* may also contain
   multiple statements separated by ";" or newlines, as long as they
   don’t contain multi-line string literals.

   To measure the execution time of the first statement, use the
   "timeit()" method.  The "repeat()" method is a convenience to call
   "timeit()" multiple times and return a list of results.

   2.6 版更變: The *stmt* and *setup* parameters can now also take
   objects that are callable without arguments.  This will embed calls
   to them in a timer function that will then be executed by
   "timeit()".  Note that the timing overhead is a little larger in
   this case because of the extra function calls.

   timeit(number=1000000)

      Time *number* executions of the main statement.  This executes
      the setup statement once, and then returns the time it takes to
      execute the main statement a number of times, measured in
      seconds as a float. The argument is the number of times through
      the loop, defaulting to one million.  The main statement, the
      setup statement and the timer function to be used are passed to
      the constructor.

      備註: By default, "timeit()" temporarily turns off *garbage
        collection* during the timing.  The advantage of this approach
        is that it makes independent timings more comparable.  This
        disadvantage is that GC may be an important component of the
        performance of the function being measured.  If so, GC can be
        re-enabled as the first statement in the *setup* string.  For
        example:

           timeit.Timer('for i in xrange(10): oct(i)', 'gc.enable()').timeit()

   repeat(repeat=3, number=1000000)

      Call "timeit()" a few times.

      This is a convenience function that calls the "timeit()"
      repeatedly, returning a list of results.  The first argument
      specifies how many times to call "timeit()".  The second
      argument specifies the *number* argument for "timeit()".

      備註: It’s tempting to calculate mean and standard deviation
        from the result vector and report these.  However, this is not
        very useful. In a typical case, the lowest value gives a lower
        bound for how fast your machine can run the given code
        snippet; higher values in the result vector are typically not
        caused by variability in Python’s speed, but by other
        processes interfering with your timing accuracy. So the
        "min()" of the result is probably the only number you should
        be interested in.  After that, you should look at the entire
        vector and apply common sense rather than statistics.

   print_exc(file=None)

      Helper to print a traceback from the timed code.

      Typical use:

         t = Timer(...)       # outside the try/except
         try:
             t.timeit(...)    # or t.repeat(...)
         except:
             t.print_exc()

      The advantage over the standard traceback is that source lines
      in the compiled template will be displayed. The optional *file*
      argument directs where the traceback is sent; it defaults to
      "sys.stderr".


26.6.3. 命令執行列介面
======================

When called as a program from the command line, the following form is
used:

   python -m timeit [-n N] [-r N] [-s S] [-t] [-c] [-h] [statement ...]

Where the following options are understood:

-n N, --number=N

   how many times to execute 『statement』

-r N, --repeat=N

   how many times to repeat the timer (default 3)

-s S, --setup=S

   statement to be executed once initially (default "pass")

-t, --time

   use "time.time()" (default on all platforms but Windows)

-c, --clock

   use "time.clock()" (default on Windows)

-v, --verbose

   print raw timing results; repeat for more digits precision

-h, --help

   print a short usage message and exit

A multi-line statement may be given by specifying each line as a
separate statement argument; indented lines are possible by enclosing
an argument in quotes and using leading spaces.  Multiple "-s" options
are treated similarly.

If "-n" is not given, a suitable number of loops is calculated by
trying successive powers of 10 until the total time is at least 0.2
seconds.

"default_timer()" measurations can be affected by other programs
running on the same machine, so the best thing to do when accurate
timing is necessary is to repeat the timing a few times and use the
best time.  The "-r" option is good for this; the default of 3
repetitions is probably enough in most cases.  On Unix, you can use
"time.clock()" to measure CPU time.

備註: There is a certain baseline overhead associated with executing
  a pass statement. The code here doesn’t try to hide it, but you
  should be aware of it.  The baseline overhead can be measured by
  invoking the program without arguments, and it might differ between
  Python versions.  Also, to fairly compare older Python versions to
  Python 2.3, you may want to use Python’s "-O" option (see
  Optimizations) for the older versions to avoid timing "SET_LINENO"
  instructions.


26.6.4. Examples
================

It is possible to provide a setup statement that is executed only once
at the beginning:

   $ python -m timeit -s 'text = "sample string"; char = "g"'  'char in text'
   10000000 loops, best of 3: 0.0877 usec per loop
   $ python -m timeit -s 'text = "sample string"; char = "g"'  'text.find(char)'
   1000000 loops, best of 3: 0.342 usec per loop

   >>> import timeit
   >>> timeit.timeit('char in text', setup='text = "sample string"; char = "g"')
   0.41440500499993504
   >>> timeit.timeit('text.find(char)', setup='text = "sample string"; char = "g"')
   1.7246671520006203

The same can be done using the "Timer" class and its methods:

   >>> import timeit
   >>> t = timeit.Timer('char in text', setup='text = "sample string"; char = "g"')
   >>> t.timeit()
   0.3955516149999312
   >>> t.repeat()
   [0.40193588800002544, 0.3960157959998014, 0.39594301399984033]

The following examples show how to time expressions that contain
multiple lines. Here we compare the cost of using "hasattr()" vs.
"try"/"except" to test for missing and present object attributes:

   $ python -m timeit 'try:' '  str.__nonzero__' 'except AttributeError:' '  pass'
   100000 loops, best of 3: 15.7 usec per loop
   $ python -m timeit 'if hasattr(str, "__nonzero__"): pass'
   100000 loops, best of 3: 4.26 usec per loop

   $ python -m timeit 'try:' '  int.__nonzero__' 'except AttributeError:' '  pass'
   1000000 loops, best of 3: 1.43 usec per loop
   $ python -m timeit 'if hasattr(int, "__nonzero__"): pass'
   100000 loops, best of 3: 2.23 usec per loop

   >>> import timeit
   >>> # attribute is missing
   >>> s = """\
   ... try:
   ...     str.__nonzero__
   ... except AttributeError:
   ...     pass
   ... """
   >>> timeit.timeit(stmt=s, number=100000)
   0.9138244460009446
   >>> s = "if hasattr(str, '__bool__'): pass"
   >>> timeit.timeit(stmt=s, number=100000)
   0.5829014980008651
   >>>
   >>> # attribute is present
   >>> s = """\
   ... try:
   ...     int.__nonzero__
   ... except AttributeError:
   ...     pass
   ... """
   >>> timeit.timeit(stmt=s, number=100000)
   0.04215312199994514
   >>> s = "if hasattr(int, '__bool__'): pass"
   >>> timeit.timeit(stmt=s, number=100000)
   0.08588060699912603

To give the "timeit" module access to functions you define, you can
pass a *setup* parameter which contains an import statement:

   def test():
       """Stupid test function"""
       L = []
       for i in range(100):
           L.append(i)

   if __name__ == '__main__':
       import timeit
       print(timeit.timeit("test()", setup="from __main__ import test"))
