line_profiler and kernprof
line_profiler is a module for doing line-by-line profiling of functions.
kernprof is a convenient script for running either line_profiler or the Python
standard library's cProfile or profile modules, depending on what is available.
They are available under a `BSD license`_.
.. _BSD license: http://packages.python.org/line_profiler/LICENSE.txt
Source releases and any binaries can be downloaded from the PyPI link.
The current release of the kernprof.py script may be downloaded separately here:
To check out the development sources, you can use Mercurial_::
$ hg clone http://www.enthought.com/~rkern/cgi-bin/hgwebdir.cgi/line_profiler
You may also download source tarballs of any snapshot from that URL.
Source releases will require a C compiler in order to build line_profiler. In
addition, Mercurial checkouts will also require Cython_ >= 0.10. Source releases
on PyPI should contain the pregenerated C sources, so Cython should not be
required in that case.
kernprof.py is a single-file pure Python script and does not require a compiler.
If you wish to use it to run cProfile and not line-by-line profiling, you may
copy it to a directory on your PATH manually and avoid trying to build any
In order to build and install line_profiler, you will simply use the standard
`build and install`_ for most Python packages::
$ python setup.py install
.. _Mercurial: http://www.selenic.com/mercurial/wiki/
.. _Cython: http://www.cython.org
.. _build and install: http://docs.python.org/install/index.html
The current profiling tools supported in Python 2.5 and later only time
function calls. This is a good first step for locating hotspots in one's program
and is frequently all one needs to do to optimize the program. However,
sometimes the cause of the hotspot is actually a single line in the function,
and that line may not be obvious from just reading the source code. These cases
are particularly frequent in scientific computing. Functions tend to be larger
(sometimes because of legitimate algorithmic complexity, sometimes because the
programmer is still trying to write FORTRAN code), and a single statement
without function calls can trigger lots of computation when using libraries like
numpy. cProfile only times explicit function calls, not special methods called
because of syntax. Consequently, a relatively slow numpy operation on large
arrays like this, ::
a[large_index_array] = some_other_large_array
is a hotspot that never gets broken out by cProfile because there is no explicit
function call in that statement.
LineProfiler can be given functions to profile, and it will time the execution
of each individual line inside those functions. In a typical workflow, one only
cares about line timings of a few functions because wading through the results
of timing every single line of code would be overwhelming. However, LineProfiler
does need to be explicitly told what functions to profile. The easiest way to
get started is to use the kernprof.py script.
If you use "kernprof.py [-l/--line-by-line] script_to_profile.py", an instance
of LineProfiler will be created and inserted into the __builtins__ namespace
with the name "profile". It has been written to be used as a decorator, so in
your script, you can decorate any function you want to profile with @profile. ::
def slow_function(a, b, c):
The default behavior of kernprof is to put the results into a binary file
script_to_profile.py.lprof . You can tell kernprof to immediately view the
formatted results at the terminal with the [-v/--view] option. Otherwise, you
can view the results later like so::
$ python -m line_profiler script_to_profile.py.lprof
For example, here are the results of profiling a single function from
a decorated version of the pystone.py benchmark (the first two lines are output
from pystone.py, not kernprof)::
Pystone(1.1) time for 50000 passes = 2.48
This machine benchmarks at 20161.3 pystones/second
Wrote profile results to pystone.py.lprof
Timer unit: 1e-06 s
Function: Proc2 at line 149
Total time: 0.606656 s
Line # Hits Time Per Hit % Time Line Contents
150 def Proc2(IntParIO):
151 50000 82003 1.6 13.5 IntLoc = IntParIO + 10
152 50000 63162 1.3 10.4 while 1:
153 50000 69065 1.4 11.4 if Char1Glob == 'A':
154 50000 66354 1.3 10.9 IntLoc = IntLoc - 1
155 50000 67263 1.3 11.1 IntParIO = IntLoc - IntGlob
156 50000 65494 1.3 10.8 EnumLoc = Ident1
157 50000 68001 1.4 11.2 if EnumLoc == Ident1:
158 50000 63739 1.3 10.5 break
159 50000 61575 1.2 10.1 return IntParIO
The source code of the function is printed with the timing information for each
line. There are six columns of information.
* Line #: The line number in the file.
* Hits: The number of times that line was executed.
* Time: The total amount of time spent executing the line in the timer's
units. In the header information before the tables, you will see a line
"Timer unit:" giving the conversion factor to seconds. It may be different
on different systems.
* Per Hit: The average amount of time spent executing the line once in the
* % Time: The percentage of time spent on that line relative to the total
amount of recorded time spent in the function.
* Line Contents: The actual source code. Note that this is always read from
disk when the formatted results are viewed, *not* when the code was
executed. If you have edited the file in the meantime, the lines will not
match up, and the formatter may not even be able to locate the function
If you are using IPython, there is an implementation of an %lprun magic command
which will let you specify functions to profile and a statement to execute. It
will also add its LineProfiler instance into the __builtins__, but typically,
you would not use it like that. You can install it by editing the IPython
configuration file ~/.ipython/ipy_user_conf.py to add the following lines::
# These two lines are standard and probably already there.
ip = IPython.ipapi.get()
# These two are the important ones.
To get usage help for %lprun, use the standard IPython help mechanism::
In : %lprun?
These two methods are expected to be the most frequent user-level ways of using
LineProfiler and will usually be the easiest. However, if you are building other
tools with LineProfiler, you will need to use the API. There are two ways to
inform LineProfiler of functions to profile: you can pass them as arguments to
the constructor or use the `add_function(f)` method after instantiation. ::
profile = LineProfiler(f, g)
LineProfiler has the same `run()`, `runctx()`, and `runcall()` methods as
cProfile.Profile as well as `enable()` and `disable()`. It should be noted,
though, that `enable()` and `disable()` are not entirely safe when nested.
Nesting is common when using LineProfiler as a decorator. In order to support
nesting, use `enable_by_count()` and `disable_by_count()`. These functions will
increment and decrement a counter and only actually enable or disable the
profiler when the count transitions from or to 0.
After profiling, the `dump_stats(filename)` method will pickle the results out
to the given file. `print_stats([stream])` will print the formatted results to
sys.stdout or whatever stream you specify. `get_stats()` will return LineStats
object, which just holds two attributes: a dictionary containing the results and
the timer unit.
kernprof also works with cProfile, its third-party incarnation lsprof, or the
pure-Python profile module depending on what is available. It has a few main
* Encapsulation of profiling concerns. You do not have to modify your script
in order to initiate profiling and save the results. Unless if you want to
use the advanced __builtins__ features, of course.
* Robust script execution. Many scripts require things like __name__,
__file__, and sys.path to be set relative to it. A naive approach at
encapsulation would just use execfile(), but many scripts which rely on
that information will fail. kernprof will set those variables correctly
before executing the script.
* Easy executable location. If you are profiling an application installed on
your PATH, you can just give the name of the executable. If kernprof does
not find the given script in the current directory, it will search your
PATH for it.
* Inserting the profiler into __builtins__. Sometimes, you just want to
profile a small part of your code. With the [-b/--builtin] argument, the
Profiler will be instantiated and inserted into your __builtins__ with the
name "profile". Like LineProfiler, it may be used as a decorator, or
enabled/disabled with `enable_by_count()` and `disable_by_count()`, or
even as a context manager with the "with profile:" statement in Python 2.5
* Pre-profiling setup. With the [-s/--setup] option, you can provide
a script which will be executed without profiling before executing the
main script. This is typically useful for cases where imports of large
libraries like wxPython or VTK are interfering with your results. If you
can modify your source code, the __builtins__ approach may be
The results of profile script_to_profile.py will be written to
script_to_profile.py.prof by default. It will be a typical marshalled file that
can be read with pstats.Stats(). They may be interactively viewed with the
$ python -m pstats script_to_profile.py.prof
Such files may also be viewed with graphical tools like kcachegrind_ through the
converter program pyprof2calltree_ or RunSnakeRun_.
.. _kcachegrind: http://kcachegrind.sourceforge.net/html/Home.html
.. _pyprof2calltree: http://pypi.python.org/pypi/pyprof2calltree/
.. _RunSnakeRun: http://www.vrplumber.com/programming/runsnakerun/
Frequently Asked Questions
* Why the name "kernprof"?
I didn't manage to come up with a meaningful name, so I named it after
* Why not use hotshot instead of line_profile?
hotshot can do line-by-line timings, too. However, it is deprecated and may
disappear from the standard library. Also, it can take a long time to
process the results while I want quick turnaround in my workflows. hotshot
pays this processing time in order to make itself minimally intrusive to the
code it is profiling. Code that does network operations, for example, may
even go down different code paths if profiling slows down execution too
much. For my use cases, and I think those of many other people, their
line-by-line profiling is not affected much by this concern.
* Why not allow using hotshot from kernprof.py?
I don't use hotshot, myself. I will accept contributions in this vein,
* The line-by-line timings don't add up when one profiled function calls
another. What's up with that?
Let's say you have function F() calling function G(), and you are using
LineProfiler on both. The total time reported for G() is less than the time
reported on the line in F() that calls G(). The reason is that I'm being
reasonably clever (and possibly too clever) in recording the times.
Basically, I try to prevent recording the time spent inside LineProfiler
doing all of the bookkeeping for each line. Each time Python's tracing
facility issues a line event (which happens just before a line actually gets
executed), LineProfiler will find two timestamps, one at the beginning
before it does anything (t_begin) and one as close to the end as possible
(t_end). Almost all of the overhead of LineProfiler's data structures
happens in between these two times.
When a line event comes in, LineProfiler finds the function it belongs to.
If it's the first line in the function, we record the line number and
*t_end* associated with the function. The next time we see a line event
belonging to that function, we take t_begin of the new event and subtract
the old t_end from it to find the amount of time spent in the old line. Then
we record the new t_end as the active line for this function. This way, we
are removing most of LineProfiler's overhead from the results. Well almost.
When one profiled function F calls another profiled function G, the line in
F that calls G basically records the total time spent executing the line,
which includes the time spent inside the profiler while inside G.
The first time this question was asked, the questioner had the G() function
call as part of a larger expression, and he wanted to try to estimate how
much time was being spent in the function as opposed to the rest of the
expression. My response was that, even if I could remove the effect, it
might still be misleading. G() might be called elsewhere, not just from the
relevant line in F(). The workaround would be to modify the code to split it
up into two lines, one which just assigns the result of G() to a temporary
variable and the other with the rest of the expression.
I am open to suggestions on how to make this more robust. Or simple
admonitions against trying to be clever.
* Why do my list comprehensions have so many hits when I use the LineProfiler?
LineProfiler records the line with the list comprehension once for each
iteration of the list comprehension.
* Why is kernprof distributed with line_profiler? It works with just cProfile,
Partly because kernprof.py is essential to using line_profiler effectively,
but mostly because I'm lazy and don't want to maintain the overhead of two
projects for modules as small as these. However, kernprof.py is
a standalone, pure Python script that can be used to do function profiling
with just the Python standard library. You may grab it and install it by
itself without line_profiler.
* Do I need a C compiler to build line_profiler? kernprof.py?
You do need a C compiler for line_profiler. kernprof.py is a pure Python
script and can be installed separately, though.
* Do I need Cython to build line_profiler?
You should not have to if you are building from a released source tarball.
It should contain the generated C sources already. If you are running into
problems, that may be a bug; let me know. If you are building from
a Mercurial checkout or snapshot, you will need Cython to generate the
C sources. You will probably need version 0.10 or higher. There is a bug in
some earlier versions in how it handles NULL PyObject* pointers.
* What version of Python do I need?
Both line_profiler and kernprof have been tested with Python 2.4 and Python
2.5. It might work with Python 2.3, and will probably work with Python 2.6.
* I get negative line timings! What's going on?
This is a known bug on Windows. I'm working on it. If you see it anywhere
else, let me know.
cProfile uses a neat "rotating trees" data structure to minimize the overhead of
looking up and recording entries. LineProfiler uses Python dictionaries and
extension objects thanks to Cython. This mostly started out as a prototype that
I wanted to play with as quickly as possible, so I passed on stealing the
rotating trees for now. As usual, I got it working, and it seems to have
acceptable performance, so I am much less motivated to use a different strategy
now. Maybe later. Contributions accepted!
Bugs and Such
If you find a bug, or a missing feature you really want added, please post to
the enthought-dev_ mailing list or email the author at
.. _enthought-dev : https://mail.enthought.com/mailman/listinfo/enthought-dev