User Manual

So you want to write algorithms that are easy to maintain as in Python and you want performance as in FORTRAN or C++? Lets give a try to Pythran! Pythran is a Python-to-c++ translator that turns Python modules into native c++11 modules. From a user point of view, you still import your module, but under the hood... There is much more happening!


Pythran is not a full Python-to-c++ converter, as is shedskin. Instead it takes a subset of the Python language and turns it into heavily templatized c++ code instantiated for your particular types.

Say hello to:

  • polymorphic functions (!)
  • lambdas
  • list comprehension
  • map, reduce and the like
  • dictionary, set, list
  • exceptions
  • file handling
  • (partial) numpy support

Say bye bye to:

  • classes
  • polymorphic variables [ but not all of them :-) ]

In a nutshell, Pythran makes it possible to write numerical algorithms in Python and to have them run faster. Nuff said.


Pythran depends on the following packages:

You also need a modern C++11 enabled compiler (e.g. g++>=4.9, clang>=3.5), that supports atomic operations (N3290) or variadic template (N2555).

Manual Installation

First get the sources:

$> git clone

Install cough numpy manually:

$> pip install --user numpy

From the source directory, run:

$> python install --user

And set your path to:

$> export PATH=$PATH:$HOME/.local/bin

It makes the pythran command available to you.

Making Sure Everything is working

The scripts automates this. The test target, as in:

python test

runs a whole (and long) validation suite (you will need to install the pytest module first to use it).

If these tests fail, you are likely missing some of the requirements. You can set site specific flags in your ~/.pythranrc, read the doc a bit further!

First Steps

To begin with, you need... a Python function in a module. Something like:

def dprod(arr0, arr1):
    return sum([x*y for x,y in zip(arr0, arr1)])

is perfect. But due to \_o< typing, arr0 and arr1 can be of any type, so Pythran needs a small hint there. Add the following line somewhere in your file, say at the top head, or right before the function definition:

#pythran export dprod(int list, int list)

This basically tells Pythran the type of the forthcoming arguments.

Afterwards, frenetically type:

$> pythran

\o/ a native module has been created and you can play with it right now, as if it where a normal module:

>>> import dprod # this imports the native version if available
>>> dprod.dprod([1,2], [3,4])

The speedup will not be terrific because of the conversion cost from Python to C++.

So let’s try again with a well-known example. Let me introduce the almighty matrix multiply!:

def zero(n,m): return [[0]*n for col in range(m)]
def matrix_multiply(m0, m1):
    new_matrix = zero(len(m0),len(m1[0]))
    for i in range(len(m0)):
        for j in range(len(m1[0])):
            for k in range(len(m1)):
                new_matrix[i][j] += m0[i][k]*m1[k][j]
    return new_matrix

This is a slightly more complex example, as a few intrinsics such as range and len are used, with a function call and even nested list comprehension. But Pythran can make its way through this. As you only want to export the matrix_multiply function, you can safely ignore the zero function and just add:

#pythran export matrix_multiply(float list list, float list list)

to the source file. Note how Pythran can combine different types and infer the resulting type. It also respects the nested list structure of Python, so you are not limited to matrices...

Enough talk, run:

$> pythran

One touch of magic wand and you have your native binary. Be amazed by the generation of a native module that runs around 20x faster than the original one. timeit approved!

But scientific computing in Python usually means Numpy. Here is a well-known Numpy snippet:

import numpy as np
def arc_distance(theta_1, phi_1, theta_2, phi_2):
    Calculates the pairwise arc distance
    between all points in vector a and b.
    temp = (np.sin((theta_2-theta_1)/2)**2
           + np.cos(theta_1)*np.cos(theta_2) * np.sin((phi_2-phi_1)/2)**2)
    distance_matrix = 2 * np.arctan2(np.sqrt(temp), np.sqrt(1-temp))
    return distance_matrix

This example uses a lot of Numpy ufunc. Pythran is reasonably good at handling such expressions. As you already know, you need to export it, giving its argument types by adding:

#pythran export arc_distance(float[], float[], float[], float[])

To the input file. You can compile it as the previous code:

$> pythran

and you’ll get a decent binary. But what you really want to do is:

$> pythran -fopenmp -march=avx

which basically tells the compiler to parallelize and vectorize loops. Then you’ll get really fast code!

Concerning Pythran specifications

The #pythran export commands are critical to Pythran. In fact if they are missing, Pythran will complain loudly (and fail miserably). So let us dive into this complex language!

There is currently only one Pythran command, the export command. Its syntax is:

#pythran export function_name(argument_type*)

where function_name is the name of a function defined in the module, and argument_type* is a comma separated list of argument types, composed of any combination of basic types and constructed types. What is a basic type? Anything that looks like a Python basic type! Constructed types are either tuples, introduced by parenthesis, like (int, (float, str)) or lists (resp. set), introduced by the list (resp. set) keyword:

argument_type = basic_type
              | (argument_type+)    # this is a tuple
              | argument_type list    # this is a list
              | argument_type set    # this is a set
              | argument_type []+    # this is a ndarray, C-style
              | argument_type [::]+    # this is a strided ndarray
              | argument_type [:,...,:]+ # this is a ndarray, Cython
              | argument_type:argument_type dict    # this is a dictionary

basic_type = bool | int | long | float | str | None
           | uint8 | uint16 | uint32 | uint64
           | int8 | int16 | int32 | int64
           | float32 | float64
           | complex64 | complex128

The same syntax can be used to export global variable (in read only mode):

#pythran export var_name

In a similar manner to the Python import statement, it’s possible to chain the export, as in:

#pythran export var_name0, var_name1, function_name(argument_type0)

Easy enough, isn’t it?


Multiline exports are supported, just use comments to protect them, as in:

#pythran export river_boa(int,
#                         float,
#                         bool)


It is in fact possible to analyse a code without specifications, but you cannot go further that generic (a.k.a. heavily templated) c++ code. Use the -e switch!

Python3 Compatibility

Pythran now supports Python3 and can be installed as a regular Python3 program. Note however that Python3 support is still in early stage and compilation failure may happen. Report them!

GIL Interaction

As Pythran translates the Python code in native code that only depends on libpython for data translation, it can release the GIL during the actual function run. And that’s what it does :-) Put an another way, you can rip some speedup at the Python level just by spawning multiple threading.Thread.

IPython Integration

The magic function %%pythran is made available to ipython users through an extension. The extension is located in the extensions/ directory and can be loaded using IPython’s magic function:

%load_ext pythran.magic

Once done, you can pythranize your code from the IPython shell:

#pythran export foo()
def foo(): print 'hello'

You can pass arguments to this magic, as in:

%%pythran(-O2 -fopenmp)
#pythran export foo()
def foo(): print 'hello'

Distutils Integration

When distributing a Python application with Pythran modules, you can either:

  • declare the module as a regular Python module. After all, they are 100% Python compatible.

  • declare them as a PythranExtension and Pythran will compile them:

    from distutils.core import setup
    # These two lines a re required to be able to use pythran in the
    import setuptools
    from pythran.dist import PythranExtension
          ext_modules=[PythranExtension("mymodule", [""])])

Advanced Usage

A failing compilation? A lust for c++ tangled code? Give a try to the -E switch that stops the compilation process right after c++ code generation, so that you can inspect it.

Want more performance? Big fan of -Ofast -march=native? Pythran _automagically_ forwards these switches to the underlying compiler! Pythran is sensible to the -DNDEBUG switch too.

Tired of typing the same compiler switches again and again? Store them in $XDG_CONFIG_HOME/.pythranrc!

Wants to try your own compiler? Update the CC and CXX fields from your pythranrc, or set the same env variables to the right compilers.

The careful reader might have noticed the -p flag from the command line. It makes it possible to define your own optimization sequence:

pythran -pConstantFodling -pmy_package.MyOptimization

runs the ConstantFolding optimization from pythran.optimizations followed by a custom optimization found in the my_package package, loaded from PYTHONPATH.

When importing a Python module, one can check for the presence of the __pythran__ variable at the module scope to see if the module has been pythranized:

import foo
if hasattr(foo, '__pythran__'):

This variable is a tuple that holds three fields:

  1. pythran’s version
  2. compilation date
  3. sha256 value of the input code

Adding OpenMP directives

OpenMP is a standard set of directives for C, C++ and FORTRAN that makes it easier to turn a sequential program into a multi-threaded one. Pythran translates OpenMP-like code annotation into OpenMP directives:

#omp parallel for reduction(+:r)
for x,y in zip(l1,l2):

OpenMP directive parsing is enabled by -fopenmp when using g++ as the back-end compiler. Be careful with the indentation. It has to be correct!

Alternatively, one can run the great:

pythran -ppythran.analysis.ParallelMaps -e

which runs a code analyzer that displays extra information concerning parallel map found in the code.

Getting Pure C++

Pythran can be used to generate raw templated C++ code, without any Python glue. To do so use the -e switch. It will turn the Python code into C++ code you can call from a C++ program. In that case there is no need for a particular Pythran specification.

Customizing Your .pythranrc

Pythran checks for a file named .pythranrc and use it to replace the site configuration. Here are a few tricks!


This section contains compiler flags configuration. For education purpose, the default linux configuration is

libs=gmp gmpxx
cflags=-std=c++11 -fno-math-errno -w -fwhole-program -fvisibility=hidden
ldflags=-fvisibility=hidden -Wl,-strip-all
CC:Path to the C compiler to use
CXX:Path to the C++ compiler to use
defines:Preprocessor definitions. Pythran is sensible to USE_BOOST_SIMD and PYTHRAN_OPENMP_MIN_ITERATION_COUNT. The former turns on Boost.simd vectorization and the latter controls the mimimal loop trip count to turn a sequential loop into a parallel loop. The default is to set USE_GMP, so that Python’s longs are represented using GMP.
undefs:Some preprocessor definitions to remove.
include_dirs:Additionnal include directories to search for headers.
cflags:Additionnal random compiler flags (-f, -O). Optimization flags generally go there. The default is to set -std=c++11 for C++11 support.
libs:Libraries to use during the link process. A typical extension is to add tcmalloc_minimal to use the allocator from
library_dirs:Extra directories to search for required libraries.
ldflags:Additionnal random linker flags.
blas:BLAS library to use. Default is empty, which lets Numpy choose it for you. If you wan to use, say, openblas, then set this to openblas.


This one contains internal configuration settings. Play with it at your own risk!

optimizations:A list of import paths pointing to transformation classes. This contains the optimization pipeline of Pythran! If you design your own optimizations, register them here!
complex_hook:Set this to True for faster and still numpy-compliant complex multiplications. Not very portable, but generally works on Linux.


Another internal setting stuff. This controls the accuracy of the typing phase. An extract from the default setting file should convince you not to touch it:


# maximum number of container access taken into account during type inference
# increasing this value inreases typing accuracy
# but slows down compilation time, to the point of making g++ crash
max_container_type = 2

# maximum number of combiner per user function
# increasing this value inreases typing accuracy
# but slows down compilation time, to the point of making g++ crash
max_combiner = 2

# set this to true to enable a costly yet more accurate type inference algorithm
# This algorithms generates code difficult to compile for g++, but not clang++
enable_two_steps_typing = False


  1. Supported compiler versions:
    • g++ version 4.9
    • clang++ version 3.5


Plenty of them! Seriously, Pythran is software, so it will crash. You may make it abort in unusual ways! And more importantly, please provide feedback to serge_sans_paille using its email, the IRC channel #pythran on FreeNode, or the mailing list