1. Nick Coghlan
  2. cpython_sandbox


cpython_sandbox / Doc / library / profile.rst

The Python Profilers

Source code: :source:`Lib/profile.py` and :source:`Lib/pstats.py`

Introduction to the profilers

:mod:`cProfile` and :mod:`profile` provide :dfn:`deterministic profiling` of Python programs. A :dfn:`profile` is a set of statistics that describes how often and for how long various parts of the program executed. These statistics can be formatted into reports via the :mod:`pstats` module.

The Python standard library provides two different implementations of the same profiling interface:

  1. :mod:`cProfile` is recommended for most users; it's a C extension with reasonable overhead that makes it suitable for profiling long-running programs. Based on :mod:`lsprof`, contributed by Brett Rosen and Ted Czotter.
  2. :mod:`profile`, a pure Python module whose interface is imitated by :mod:`cProfile`, but which adds significant overhead to profiled programs. If you're trying to extend the profiler in some way, the task might be easier with this module.


The profiler modules are designed to provide an execution profile for a given program, not for benchmarking purposes (for that, there is :mod:`timeit` for reasonably accurate results). This particularly applies to benchmarking Python code against C code: the profilers introduce overhead for Python code, but not for C-level functions, and so the C code would seem faster than any Python one.

Instant User's Manual

This section is provided for users that "don't want to read the manual." It provides a very brief overview, and allows a user to rapidly perform profiling on an existing application.

To profile a function that takes a single argument, you can do:

import cProfile
import re

(Use :mod:`profile` instead of :mod:`cProfile` if the latter is not available on your system.)

The above action would run :func:`re.compile` and print profile results like the following:

      197 function calls (192 primitive calls) in 0.002 seconds

Ordered by: standard name

ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     1    0.000    0.000    0.001    0.001 <string>:1(<module>)
     1    0.000    0.000    0.001    0.001 re.py:212(compile)
     1    0.000    0.000    0.001    0.001 re.py:268(_compile)
     1    0.000    0.000    0.000    0.000 sre_compile.py:172(_compile_charset)
     1    0.000    0.000    0.000    0.000 sre_compile.py:201(_optimize_charset)
     4    0.000    0.000    0.000    0.000 sre_compile.py:25(_identityfunction)
   3/1    0.000    0.000    0.000    0.000 sre_compile.py:33(_compile)

The first line indicates that 197 calls were monitored. Of those calls, 192 were :dfn:`primitive`, meaning that the call was not induced via recursion. The next line: Ordered by: standard name, indicates that the text string in the far right column was used to sort the output. The column headings include:

for the number of calls,
for the total time spent in the given function (and excluding time made in calls to sub-functions)
is the quotient of tottime divided by ncalls
is the cumulative time spent in this and all subfunctions (from invocation till exit). This figure is accurate even for recursive functions.
is the quotient of cumtime divided by primitive calls
provides the respective data of each function

When there are two numbers in the first column (for example 3/1), it means that the function recursed. The second value is the number of primitive calls and the former is the total number of calls. Note that when the function does not recurse, these two values are the same, and only the single figure is printed.

Instead of printing the output at the end of the profile run, you can save the results to a file by specifying a filename to the :func:`run` function:

import cProfile
import re
cProfile.run('re.compile("foo|bar")', 'restats')

The :class:`pstats.Stats` class reads profile results from a file and formats them in various ways.

The file :mod:`cProfile` can also be invoked as a script to profile another script. For example:

python -m cProfile [-o output_file] [-s sort_order] myscript.py

-o writes the profile results to a file instead of to stdout

-s specifies one of the :func:`~pstats.Stats.sort_stats` sort values to sort the output by. This only applies when -o is not supplied.

The :mod:`pstats` module's :class:`~pstats.Stats` class has a variety of methods for manipulating and printing the data saved into a profile results file:

import pstats
p = pstats.Stats('restats')

The :meth:`~pstats.Stats.strip_dirs` method removed the extraneous path from all the module names. The :meth:`~pstats.Stats.sort_stats` method sorted all the entries according to the standard module/line/name string that is printed. The :meth:`~pstats.Stats.print_stats` method printed out all the statistics. You might try the following sort calls:


The first call will actually sort the list by function name, and the second call will print out the statistics. The following are some interesting calls to experiment with:


This sorts the profile by cumulative time in a function, and then only prints the ten most significant lines. If you want to understand what algorithms are taking time, the above line is what you would use.

If you were looking to see what functions were looping a lot, and taking a lot of time, you would do:


to sort according to time spent within each function, and then print the statistics for the top ten functions.

You might also try:


This will sort all the statistics by file name, and then print out statistics for only the class init methods (since they are spelled with __init__ in them). As one final example, you could try:

p.sort_stats('time', 'cum').print_stats(.5, 'init')

This line sorts statistics with a primary key of time, and a secondary key of cumulative time, and then prints out some of the statistics. To be specific, the list is first culled down to 50% (re: .5) of its original size, then only lines containing init are maintained, and that sub-sub-list is printed.

If you wondered what functions called the above functions, you could now (p is still sorted according to the last criteria) do:

p.print_callers(.5, 'init')

and you would get a list of callers for each of the listed functions.

If you want more functionality, you're going to have to read the manual, or guess what the following functions do:


Invoked as a script, the :mod:`pstats` module is a statistics browser for reading and examining profile dumps. It has a simple line-oriented interface (implemented using :mod:`cmd`) and interactive help.

:mod:`profile` and :mod:`cProfile` Module Reference

Both the :mod:`profile` and :mod:`cProfile` modules provide the following functions:

This class is normally only used if more precise control over profiling is needed than what the :func:`cProfile.run` function provides.

A custom timer can be supplied for measuring how long code takes to run via the timer argument. This must be a function that returns a single number representing the current time. If the number is an integer, the timeunit specifies a multiplier that specifies the duration of each unit of time. For example, if the timer returns times measured in thousands of seconds, the time unit would be .001.

Directly using the :class:`Profile` class allows formatting profile results without writing the profile data to a file:

import cProfile, pstats, io
pr = cProfile.Profile()
... do something ...
s = io.StringIO()
ps = pstats.Stats(pr, stream=s)

The :class:`Stats` Class

Analysis of the profiler data is done using the :class:`~pstats.Stats` class.

This class constructor creates an instance of a "statistics object" from a filename (or list of filenames) or from a :class:`Profile` instance. Output will be printed to the stream specified by stream.

The file selected by the above constructor must have been created by the corresponding version of :mod:`profile` or :mod:`cProfile`. To be specific, there is no file compatibility guaranteed with future versions of this profiler, and there is no compatibility with files produced by other profilers. If several files are provided, all the statistics for identical functions will be coalesced, so that an overall view of several processes can be considered in a single report. If additional files need to be combined with data in an existing :class:`~pstats.Stats` object, the :meth:`~pstats.Stats.add` method can be used.

Instead of reading the profile data from a file, a :class:`cProfile.Profile` or :class:`profile.Profile` object can be used as the profile data source.

:class:`Stats` objects have the following methods:

What Is Deterministic Profiling?

:dfn:`Deterministic profiling` is meant to reflect the fact that all function call, function return, and exception events are monitored, and precise timings are made for the intervals between these events (during which time the user's code is executing). In contrast, :dfn:`statistical profiling` (which is not done by this module) randomly samples the effective instruction pointer, and deduces where time is being spent. The latter technique traditionally involves less overhead (as the code does not need to be instrumented), but provides only relative indications of where time is being spent.

In Python, since there is an interpreter active during execution, the presence of instrumented code is not required to do deterministic profiling. Python automatically provides a :dfn:`hook` (optional callback) for each event. In addition, the interpreted nature of Python tends to add so much overhead to execution, that deterministic profiling tends to only add small processing overhead in typical applications. The result is that deterministic profiling is not that expensive, yet provides extensive run time statistics about the execution of a Python program.

Call count statistics can be used to identify bugs in code (surprising counts), and to identify possible inline-expansion points (high call counts). Internal time statistics can be used to identify "hot loops" that should be carefully optimized. Cumulative time statistics should be used to identify high level errors in the selection of algorithms. Note that the unusual handling of cumulative times in this profiler allows statistics for recursive implementations of algorithms to be directly compared to iterative implementations.


One limitation has to do with accuracy of timing information. There is a fundamental problem with deterministic profilers involving accuracy. The most obvious restriction is that the underlying "clock" is only ticking at a rate (typically) of about .001 seconds. Hence no measurements will be more accurate than the underlying clock. If enough measurements are taken, then the "error" will tend to average out. Unfortunately, removing this first error induces a second source of error.

The second problem is that it "takes a while" from when an event is dispatched until the profiler's call to get the time actually gets the state of the clock. Similarly, there is a certain lag when exiting the profiler event handler from the time that the clock's value was obtained (and then squirreled away), until the user's code is once again executing. As a result, functions that are called many times, or call many functions, will typically accumulate this error. The error that accumulates in this fashion is typically less than the accuracy of the clock (less than one clock tick), but it can accumulate and become very significant.

The problem is more important with :mod:`profile` than with the lower-overhead :mod:`cProfile`. For this reason, :mod:`profile` provides a means of calibrating itself for a given platform so that this error can be probabilistically (on the average) removed. After the profiler is calibrated, it will be more accurate (in a least square sense), but it will sometimes produce negative numbers (when call counts are exceptionally low, and the gods of probability work against you :-). ) Do not be alarmed by negative numbers in the profile. They should only appear if you have calibrated your profiler, and the results are actually better than without calibration.


The profiler of the :mod:`profile` module subtracts a constant from each event handling time to compensate for the overhead of calling the time function, and socking away the results. By default, the constant is 0. The following procedure can be used to obtain a better constant for a given platform (see :ref:`profile-limitations`).

import profile
pr = profile.Profile()
for i in range(5):

The method executes the number of Python calls given by the argument, directly and again under the profiler, measuring the time for both. It then computes the hidden overhead per profiler event, and returns that as a float. For example, on a 1.8Ghz Intel Core i5 running Mac OS X, and using Python's time.clock() as the timer, the magical number is about 4.04e-6.

The object of this exercise is to get a fairly consistent result. If your computer is very fast, or your timer function has poor resolution, you might have to pass 100000, or even 1000000, to get consistent results.

When you have a consistent answer, there are three ways you can use it:

import profile

# 1. Apply computed bias to all Profile instances created hereafter.
profile.Profile.bias = your_computed_bias

# 2. Apply computed bias to a specific Profile instance.
pr = profile.Profile()
pr.bias = your_computed_bias

# 3. Specify computed bias in instance constructor.
pr = profile.Profile(bias=your_computed_bias)

If you have a choice, you are better off choosing a smaller constant, and then your results will "less often" show up as negative in profile statistics.

Using a customer timer

If you want to change how current time is determined (for example, to force use of wall-clock time or elapsed process time), pass the timing function you want to the :class:`Profile` class constructor:

pr = profile.Profile(your_time_func)

The resulting profiler will then call your_time_func. Depending on whether you are using :class:`profile.Profile` or :class:`cProfile.Profile`, your_time_func's return value will be interpreted differently:


your_time_func should return a single number, or a list of numbers whose sum is the current time (like what :func:`os.times` returns). If the function returns a single time number, or the list of returned numbers has length 2, then you will get an especially fast version of the dispatch routine.

Be warned that you should calibrate the profiler class for the timer function that you choose (see :ref:`profile-calibration`). For most machines, a timer that returns a lone integer value will provide the best results in terms of low overhead during profiling. (:func:`os.times` is pretty bad, as it returns a tuple of floating point values). If you want to substitute a better timer in the cleanest fashion, derive a class and hardwire a replacement dispatch method that best handles your timer call, along with the appropriate calibration constant.


your_time_func should return a single number. If it returns integers, you can also invoke the class constructor with a second argument specifying the real duration of one unit of time. For example, if your_integer_time_func returns times measured in thousands of seconds, you would construct the :class:`Profile` instance as follows:

pr = cProfile.Profile(your_integer_time_func, 0.001)

As the :mod:`cProfile.Profile` class cannot be calibrated, custom timer functions should be used with care and should be as fast as possible. For the best results with a custom timer, it might be necessary to hard-code it in the C source of the internal :mod:`_lsprof` module.

Python 3.3 adds several new functions in :mod:`time` that can be used to make precise measurements of process or wall-clock time. For example, see :func:`time.perf_counter`.