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319 lines
11 KiB
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319 lines
11 KiB
ReStructuredText
:mod:`timeit` --- Measure execution time of small code snippets
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===============================================================
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.. module:: timeit
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:synopsis: Measure the execution time of small code snippets.
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.. index::
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single: Benchmarking
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single: Performance
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**Source code:** :source:`Lib/timeit.py`
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--------------
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This module provides a simple way to time small bits of Python code. It has both
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a :ref:`command-line-interface` as well as a :ref:`callable <python-interface>`
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one. It avoids a number of common traps for measuring execution times.
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See also Tim Peters' introduction to the "Algorithms" chapter in the *Python
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Cookbook*, published by O'Reilly.
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Basic Examples
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--------------
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The following example shows how the :ref:`command-line-interface`
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can be used to compare three different expressions:
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.. code-block:: sh
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$ python -m timeit '"-".join(str(n) for n in range(100))'
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10000 loops, best of 3: 40.3 usec per loop
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$ python -m timeit '"-".join([str(n) for n in range(100)])'
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10000 loops, best of 3: 33.4 usec per loop
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$ python -m timeit '"-".join(map(str, range(100)))'
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10000 loops, best of 3: 25.2 usec per loop
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This can be achieved from the :ref:`python-interface` with::
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>>> import timeit
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>>> timeit.timeit('"-".join(str(n) for n in range(100))', number=10000)
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0.8187260627746582
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>>> timeit.timeit('"-".join([str(n) for n in range(100)])', number=10000)
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0.7288308143615723
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>>> timeit.timeit('"-".join(map(str, range(100)))', number=10000)
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0.5858950614929199
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Note however that :mod:`timeit` will automatically determine the number of
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repetitions only when the command-line interface is used. In the
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:ref:`timeit-examples` section you can find more advanced examples.
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.. _python-interface:
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Python Interface
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----------------
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The module defines three convenience functions and a public class:
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.. function:: timeit(stmt='pass', setup='pass', timer=<default timer>, number=1000000)
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Create a :class:`Timer` instance with the given statement, *setup* code and
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*timer* function and run its :meth:`.timeit` method with *number* executions.
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.. function:: repeat(stmt='pass', setup='pass', timer=<default timer>, repeat=3, number=1000000)
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Create a :class:`Timer` instance with the given statement, *setup* code and
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*timer* function and run its :meth:`.repeat` method with the given *repeat*
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count and *number* executions.
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.. function:: default_timer()
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The default timer, which is always :func:`time.perf_counter`.
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.. versionchanged:: 3.3
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:func:`time.perf_counter` is now the default timer.
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.. class:: Timer(stmt='pass', setup='pass', timer=<timer function>)
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Class for timing execution speed of small code snippets.
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The constructor takes a statement to be timed, an additional statement used
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for setup, and a timer function. Both statements default to ``'pass'``;
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the timer function is platform-dependent (see the module doc string).
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*stmt* and *setup* may also contain multiple statements separated by ``;``
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or newlines, as long as they don't contain multi-line string literals.
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To measure the execution time of the first statement, use the :meth:`.timeit`
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method. The :meth:`.repeat` method is a convenience to call :meth:`.timeit`
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multiple times and return a list of results.
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The *stmt* and *setup* parameters can also take objects that are callable
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without arguments. This will embed calls to them in a timer function that
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will then be executed by :meth:`.timeit`. Note that the timing overhead is a
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little larger in this case because of the extra function calls.
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.. method:: Timer.timeit(number=1000000)
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Time *number* executions of the main statement. This executes the setup
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statement once, and then returns the time it takes to execute the main
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statement a number of times, measured in seconds as a float.
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The argument is the number of times through the loop, defaulting to one
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million. The main statement, the setup statement and the timer function
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to be used are passed to the constructor.
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.. note::
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By default, :meth:`.timeit` temporarily turns off :term:`garbage
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collection` during the timing. The advantage of this approach is that
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it makes independent timings more comparable. This disadvantage is
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that GC may be an important component of the performance of the
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function being measured. If so, GC can be re-enabled as the first
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statement in the *setup* string. For example::
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timeit.Timer('for i in range(10): oct(i)', 'gc.enable()').timeit()
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.. method:: Timer.repeat(repeat=3, number=1000000)
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Call :meth:`.timeit` a few times.
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This is a convenience function that calls the :meth:`.timeit` repeatedly,
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returning a list of results. The first argument specifies how many times
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to call :meth:`.timeit`. The second argument specifies the *number*
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argument for :meth:`.timeit`.
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.. note::
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It's tempting to calculate mean and standard deviation from the result
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vector and report these. However, this is not very useful.
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In a typical case, the lowest value gives a lower bound for how fast
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your machine can run the given code snippet; higher values in the
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result vector are typically not caused by variability in Python's
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speed, but by other processes interfering with your timing accuracy.
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So the :func:`min` of the result is probably the only number you
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should be interested in. After that, you should look at the entire
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vector and apply common sense rather than statistics.
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.. method:: Timer.print_exc(file=None)
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Helper to print a traceback from the timed code.
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Typical use::
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t = Timer(...) # outside the try/except
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try:
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t.timeit(...) # or t.repeat(...)
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except Exception:
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t.print_exc()
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The advantage over the standard traceback is that source lines in the
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compiled template will be displayed. The optional *file* argument directs
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where the traceback is sent; it defaults to :data:`sys.stderr`.
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.. _command-line-interface:
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Command-Line Interface
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----------------------
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When called as a program from the command line, the following form is used::
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python -m timeit [-n N] [-r N] [-s S] [-t] [-c] [-h] [statement ...]
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Where the following options are understood:
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.. program:: timeit
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.. cmdoption:: -n N, --number=N
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how many times to execute 'statement'
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.. cmdoption:: -r N, --repeat=N
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how many times to repeat the timer (default 3)
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.. cmdoption:: -s S, --setup=S
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statement to be executed once initially (default ``pass``)
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.. cmdoption:: -p, --process
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measure process time, not wallclock time, using :func:`time.process_time`
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instead of :func:`time.perf_counter`, which is the default
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.. versionadded:: 3.3
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.. cmdoption:: -t, --time
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use :func:`time.time` (deprecated)
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.. cmdoption:: -c, --clock
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use :func:`time.clock` (deprecated)
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.. cmdoption:: -v, --verbose
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print raw timing results; repeat for more digits precision
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.. cmdoption:: -h, --help
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print a short usage message and exit
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A multi-line statement may be given by specifying each line as a separate
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statement argument; indented lines are possible by enclosing an argument in
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quotes and using leading spaces. Multiple :option:`-s` options are treated
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similarly.
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If :option:`-n` is not given, a suitable number of loops is calculated by trying
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successive powers of 10 until the total time is at least 0.2 seconds.
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:func:`default_timer` measurements can be affected by other programs running on
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the same machine, so the best thing to do when accurate timing is necessary is
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to repeat the timing a few times and use the best time. The :option:`-r`
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option is good for this; the default of 3 repetitions is probably enough in
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most cases. You can use :func:`time.process_time` to measure CPU time.
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.. note::
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There is a certain baseline overhead associated with executing a pass statement.
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The code here doesn't try to hide it, but you should be aware of it. The
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baseline overhead can be measured by invoking the program without arguments,
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and it might differ between Python versions.
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.. _timeit-examples:
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Examples
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--------
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It is possible to provide a setup statement that is executed only once at the beginning:
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.. code-block:: sh
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$ python -m timeit -s 'text = "sample string"; char = "g"' 'char in text'
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10000000 loops, best of 3: 0.0877 usec per loop
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$ python -m timeit -s 'text = "sample string"; char = "g"' 'text.find(char)'
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1000000 loops, best of 3: 0.342 usec per loop
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::
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>>> import timeit
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>>> timeit.timeit('char in text', setup='text = "sample string"; char = "g"')
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0.41440500499993504
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>>> timeit.timeit('text.find(char)', setup='text = "sample string"; char = "g"')
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1.7246671520006203
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The same can be done using the :class:`Timer` class and its methods::
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>>> import timeit
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>>> t = timeit.Timer('char in text', setup='text = "sample string"; char = "g"')
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>>> t.timeit()
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0.3955516149999312
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>>> t.repeat()
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[0.40193588800002544, 0.3960157959998014, 0.39594301399984033]
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The following examples show how to time expressions that contain multiple lines.
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Here we compare the cost of using :func:`hasattr` vs. :keyword:`try`/:keyword:`except`
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to test for missing and present object attributes:
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.. code-block:: sh
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$ python -m timeit 'try:' ' str.__bool__' 'except AttributeError:' ' pass'
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100000 loops, best of 3: 15.7 usec per loop
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$ python -m timeit 'if hasattr(str, "__bool__"): pass'
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100000 loops, best of 3: 4.26 usec per loop
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$ python -m timeit 'try:' ' int.__bool__' 'except AttributeError:' ' pass'
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1000000 loops, best of 3: 1.43 usec per loop
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$ python -m timeit 'if hasattr(int, "__bool__"): pass'
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100000 loops, best of 3: 2.23 usec per loop
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::
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>>> import timeit
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>>> # attribute is missing
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>>> s = """\
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... try:
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... str.__bool__
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... except AttributeError:
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... pass
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... """
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>>> timeit.timeit(stmt=s, number=100000)
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0.9138244460009446
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>>> s = "if hasattr(str, '__bool__'): pass"
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>>> timeit.timeit(stmt=s, number=100000)
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0.5829014980008651
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>>>
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>>> # attribute is present
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>>> s = """\
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... try:
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... int.__bool__
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... except AttributeError:
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... pass
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... """
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>>> timeit.timeit(stmt=s, number=100000)
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0.04215312199994514
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>>> s = "if hasattr(int, '__bool__'): pass"
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>>> timeit.timeit(stmt=s, number=100000)
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0.08588060699912603
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To give the :mod:`timeit` module access to functions you define, you can pass a
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*setup* parameter which contains an import statement::
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def test():
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"""Stupid test function"""
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L = [i for i in range(100)]
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if __name__ == '__main__':
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import timeit
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print(timeit.timeit("test()", setup="from __main__ import test"))
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