<chapter name="Python Interface"> 
 
<h2>Python Interface</h2> 
 
<a href="https://www.python.org/" target="_top">Python</a> is a 
powerful, high-level interpreted language that is widely used within 
the particle physics community. It is particularly useful as an 
interactive language that provides fast proto-typing of code. A Python 
interface to PYTHIA is available. This interface is automatically 
generated with <a href="https://github.com/RosettaCommons/binder" 
target="_top">Binder</a> using the <a 
href="https://github.com/pybind/pybind11" target="_top">PyBind11</a> 
template library. Please note that this interface has not been 
extensively tested and consequently issues may arise. If bugs are 
found or additional features are required, as usual, please report 
them to the PYTHIA authors. This interface is a significant departure 
from the previous interface generated with SWIG, which does not 
support C++11. 
 
<p/> 
A simplified interface ships with PYTHIA, which is intended to meet 
the needs of most users while remaining lightweight. Note, not all 
PYTHIA classes are available through this interface. It is also 
possible for users to automatically generate their own interface. This 
is particularly useful if users modify the PYTHIA source code and need 
to update the interface. When generating the interface there are three 
options: the simplified interface, a full interface, and a user 
defined interface. The process of generating these interfaces is 
described in more detail below. 
 
<h3>Simplified Interface</h3> 
 
An attempt has been made to provide the everyday functionality of 
PYTHIA through the simplified interface. However, if classes or 
methods appear to be missing, please contact the PYTHIA authors; 
additional functionality can be included upon request. The following 
list highlights the available classes and methods in this interface, 
roughly categorized by function. Note that <code>help(pythia8)</code> 
will return all the available classes and methods in the interface. 
<ul> 
<li>top level: <code>Pythia</code></li> 
 
<li>event access: <code>Event</code>, <code>Particle</code>, 
<code>Vec4</code>, <code>RotBstMatrix</code></li> 
 
<li>particle data: <code>ParticleData</code>, 
<code>ParticleDataEntry</code>, <code>DecayChannel</code></li> 
 
<li>settings and information: <code>Info</code>, 
<code>Settings</code>, <code>Flag</code>, <code>Mode</code>, 
<code>Parm</code>, <code>Word</code>, <code>FVec</code>, 
<code>MVec</code>, <code>PVec</code>, <code>WVec</code></li> 
 
<li>user interface pointers: <code>Rndm</code>, <code>PDF</code>, 
<code>DecayHandler</code>, <code>RndmEngine</code>, 
<code>UserHooks</code>, <code>MergingHooks</code>, 
<code>BeamShape</code>, <code>SigmaProcess</code>, 
<code>TimeShower</code>, <code>SpaceShower</code>, 
<code>HeavyIons</code>, <code>PartonVertex</code></li> 
 
<li>analysis tools: <code>Hist</code>, <code>HistPlot</code>, 
<code>SlowJet</code>, <code>SlowJetHook</code></li> <li>kinematic 
functions: <code>m</code>, <code>m2</code>, <code>dot3</code>, 
<code>cross3</code>, <code>cross4</code>, <code>theta</code>, 
<code>costheta</code>, <code>phi</code>, <code>cosphi</code>, 
<code>RRapPhi</code>, <code>REtaPhi</code></li> 
</ul> 
 
The following points highlight some of the features of the interface. 
<ul> 
<li>Documentation through the built-in help function in Python 
provides the relevant mappings between the Python interface and the 
C++ code. For documentation on the purpose of the code, see the user 
HTML manual and Doxygen</li> 
<li>Templated C++ types are returned as native Python types, 
e.g. vectors are returned as lists, etc. Similarly, native Python 
types can be passed as arguments and are converted to their equivalent 
C++ templates</li> 
<li>No difference is made between passing by reference or pointer; 
methods in C++ which take a pointer as an argument in Python 
simply take the object, e.g. <code>foo(Vec4*)</code> and 
<code>foo(Vec4&amp;)</code> are the same in the Python interface.</li> 
<li>All operators defined in C++, e.g. <code>Vec4*double</code> are 
available. Note that reverse operators are not, 
e.g. <code>double*Vec4</code>.</li> 
<li>Classes with defined <code>[]</code> operators are iterable, using 
standard Python iteration, e.g. <code>for prt in 
pythia.event</code>.</li> 
<li>Classes with a <code>&lt;&lt;</code> operator can be printed via 
the built-in print function in Python. Note this means that a string 
representation via <code>str</code> is also available for these 
classes in Python.</li> 
<li>Derived classes in Python can be passed back to PYTHIA, and should 
be available for all classes. See <code>main10.py</code> for a direct 
Python translation of the C++ <code>main10.cc</code> example which 
uses a derived class from the UserHooks class to veto events.</li> 
<li>The constant <code>Pythia::info</code> member is available as 
<code>Pythia::infoPython</code>. Note, this creates a new instance of 
the <code>Info</code> class, and so needs to be called each time the 
information might be updated.</li> 
</ul> 
 
<h3>Limitations</h3> 
 
In general, most code using PYTHIA implemented through C++ is also 
possible in the Python interface. There are a number of issues, such 
as passing streams, which cannot be handled in the Python 
interface. Additionally, protected members of classes are exposed as 
fully public members in the Python interface. 
 
<h3>Installation</h3> 
 
To install the Python interface, the Python system header 
<code>Python.h</code> must be available. By default when configuring 
using <code>--with-python</code> the system <code>python-config</code> 
script will be used to determine the location of 
<code>Python.h</code>. In some cases, users might wish to use a 
different version of Python, and so <code>--with-python-config</code> 
can be used to set the Python configuration script used to pick up 
paths. Alternatively, the directory containing <code>Python.h</code> 
can be set manually with the option 
<code>--with-python-include</code>. Some example configurations could be 
as follows, 
<pre> 
    ./configure --with-python 
    ./configure --with-python-config=python3-config 
    ./configure --with-python-include=/usr/include/python2.7 
</pre> 
where the Python configuration script and path must be changed 
accordingly for the local system. 
 
<p/> 
After configuring the Python interface for PYTHIA to be built and 
running <code>make</code> as usual, the following files should be 
generated in the directory <code>lib</code>. 
<ul> 
<li><code>pythia8.so</code>: compiled Python module.</li> 
<li><code>libpythia8.[so,dylib]</code>: the standard shared PYTHIA 
library.</li> 
</ul> 
 
<p/> 
To ensure that the <code>pythia8.so</code> module is available to 
Python, the system variable <code>PYTHONPATH</code> should be set similar to 
<pre> 
    export PYTHONPATH=$(PREFIX_LIB):$PYTHONPATH 
</pre> 
where <code>PREFIX_LIB</code> is the directory <code>lib</code> which 
contains <code>pythia8.so</code>. Generally, the library paths should 
be set correctly, but it also may be necessary to set 
<pre> 
    export LD_LIBRARY_PATH=$(PREFIX_LIB):$LD_LIBRARY_PATH 
</pre> 
where <code>DYLD</code> should be substituted for <code>LD</code> in 
OS X. Alternatively, it is also possible to define the Python path 
from within Python, as is done within the provided examples. Note that 
the Python module is always compiled as <code>pythia8.so</code>, even 
in OS X. This is because older versions of Python in OS X do not 
correctly recognize modules ending with the <code>dylib</code> suffix. 
 
<h3>Examples</h3> 
 
To use the Python interface for PYTHIA, start Python 
and <code>import pythia8</code>. The provided examples can be run 
by <code>python mainXX.py</code> where <code>XX</code> is the number 
of the example. 
 
<h3>Interface Generation</h3> 
 
A script for automatically generating the Python interface, 
<code>generate</code>, is available in <code>plugins/python/</code> 
and should be run from this directory. This script requires that the 
user has Docker installed. A small Docker image of roughly 80 MB will 
be pulled to run the necessary generation code. There are a number of 
command line arguments which can be passed. 
<ul> 
<li><code>--help</code> prints the help message for the script.</li> 
<li><code>--part</code> generates the simplified interface. When 
<code>generate</code> is run without arguments, then this is the 
default behaviour. Note that this requires running the interface 
generation twice. The first time the required classes are interfaced, 
the second time additional classes are removed.</li> 
<li><code>--full</code> generates the full interface.</li> 
<li><code>--user=FILE</code> generates a user defined interface using 
the Binder configuration given by <code>FILE</code>.</li> 
<li><code>--debug</code> can be specified with any of the options 
above and stops the temporary files used in generation from being 
removed.</li> 
</ul> 
Whenever PYTHIA headers are modified, the Python interface is 
invalidated, and so this automatic generation is particularly useful 
in such a situation. An example of generating the full Python 
interface is as follows. 
<pre> 
    cd plugins/python 
    ./generate --full 
</pre> 
 
</chapter> 
 
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