Installation
************

The LAMMPS Python module enables calling the :ref:`LAMMPS C library API
<lammps_c_api>` from Python by dynamically loading functions in the
LAMMPS shared library through the Python `ctypes <ctypes_>`_
module.  Because of the dynamic loading, it is required that LAMMPS is
compiled in :ref:`"shared" mode <exe>`.

.. versionchanged:: 2Apr2025

LAMMPS currently only supports Python version 3.6 or later.

Two components are necessary for Python to be able to invoke LAMMPS code:

* The LAMMPS Python Package (``lammps``) from the ``python`` folder
* The LAMMPS Shared Library (``liblammps.so``, ``liblammps.dylib`` or
  ``liblammps.dll``) from the folder where you compiled LAMMPS.

.. _ctypes: https://docs.python.org/3/library/ctypes.html

.. _python_install_guides:

Installing the LAMMPS Python Module and Shared Library
======================================================

Making LAMMPS usable within Python and vice versa requires putting the
LAMMPS Python package (``lammps``) into a location where the Python
interpreter can find it and installing the LAMMPS shared library into a
folder that the dynamic loader searches or inside of the installed
``lammps`` package folder.  There are multiple ways to achieve this.

#. Install both components into a Python ``site-packages`` folder, either
   system-wide or in the corresponding user-specific folder. This way no
   additional environment variables need to be set, but the shared
   library is otherwise not accessible.

#. Do an installation into a virtual environment.

#. Leave the files where they are in the source/development tree and
   adjust some environment variables.

.. tabs::

   .. tab:: Python package

      Compile LAMMPS with either :doc:`CMake <Build_cmake>` or the
      :doc:`traditional make <Build_make>` procedure in :ref:`shared
      mode <exe>`.  After compilation has finished, type (in the
      compilation folder):

      .. code-block:: bash

         make install-python

      This will try to build a so-called (binary) wheel file, a
      compressed binary python package and then install it with the
      python package manager 'pip'.  Installation will be attempted into
      a system-wide ``site-packages`` folder and if that fails into the
      corresponding folder in the user's home directory.  For a
      system-wide installation you usually would have to gain superuser
      privilege first, e.g. though ``sudo``

      +------------------------+----------------------------------------------------------+-------------------------------------------------------------+
      | File                   | Location                                                 | Notes                                                       |
      +========================+==========================================================+=============================================================+
      | LAMMPS Python package  | * ``$HOME/.local/lib/pythonX.Y/site-packages/lammps``    | ``X.Y`` depends on the installed Python version             |
      +------------------------+----------------------------------------------------------+-------------------------------------------------------------+
      | LAMMPS shared library  | * ``$HOME/.local/lib/pythonX.Y/site-packages/lammps``    | ``X.Y`` depends on the installed Python version             |
      +------------------------+----------------------------------------------------------+-------------------------------------------------------------+

      For a system-wide installation those folders would then become.

      +------------------------+-------------------------------------------------+-------------------------------------------------------------+
      | File                   | Location                                        | Notes                                                       |
      +========================+=================================================+=============================================================+
      | LAMMPS Python package  | * ``/usr/lib/pythonX.Y/site-packages/lammps``   | ``X.Y`` depends on the installed Python version             |
      +------------------------+-------------------------------------------------+-------------------------------------------------------------+
      | LAMMPS shared library  | * ``/usr/lib/pythonX.Y/site-packages/lammps``   | ``X.Y`` depends on the installed Python version             |
      +------------------------+-------------------------------------------------+-------------------------------------------------------------+

      No environment variables need to be set for those, as those
      folders are searched by default by Python or the LAMMPS Python
      package.

      .. versionchanged:: 24Mar2022

      .. note::

            If there is an existing installation of the LAMMPS python
            module, ``make install-python`` will try to update it.
            However, that will fail if the older version of the module
            was installed by LAMMPS versions until 17Feb2022.  Those
            were using the distutils package, which does not create a
            "manifest" that allows a clean uninstall.  The ``make
            install-python`` command will always produce a
            lammps-<version>-<python>-<abi>-<os>-<arch>.whl file (the
            'wheel'). And this file can be later installed directly with
            ``python -m pip install <wheel file>.whl`` without having to
            type ``make install-python`` again and repeating the build
            step, too.

      For the traditional make process you can override the python
      version to version x.y when calling ``make`` with
      ``PYTHON=pythonX.Y``.  For a CMake based compilation this choice
      has to be made during the CMake configuration step.

      If the default settings of ``make install-python`` are not what you want,
      you can invoke ``install.py`` from the ``python`` directory manually as

      .. code-block:: bash

         python3 install.py -p <python package> -l <shared library> -v <version.h file> [-n] [-f]

      * The ``-p`` flag argument is the full path to the
        ``python/lammps`` folder to be installed,
      * the ``-l`` flag argument is the full path to the LAMMPS shared
        library file to be installed,
      * the ``-v`` flag argument is the full path to the ``src/version.h`` file
      * the optional ``-n`` flag instructs the script to only build a
        wheel file but not attempt to install it (default is to try installing),
      * the optional ``-w`` flag argument is the path to a folder where
        to store the resulting wheel file (default is the current folder)
      * and the optional ``-f`` argument instructs the script to force
        installation even if pip would otherwise refuse installation
        with an :ref:`error about externally managed environments
        <externally_managed>`. The Python developers recommend to not
        augment a Python installation with custom packages, both at the
        user and the system level, and advise to use virtual
        environments instead.  Some recent Linux distributions enforce
        that recommendation by default.

      Example command line for building only the wheel after building
      LAMMPS with ``cmake`` in the folder ``build``:

      .. code-block:: bash

         python3 python/install.py -n -p python/lammps -l build/liblammps.so -v src/version.h -w build


   .. tab:: Virtual environment

      A virtual environment is a minimal Python installation inside of a
      folder.  It allows isolating and customizing a Python environment
      that is mostly independent from a user or system installation.
      For the core Python environment, it uses symbolic links to the
      system installation and thus it can be set up quickly and will not
      take up much disk space.  This gives you the flexibility to
      install (newer/different) versions of Python packages that would
      potentially conflict with already installed system packages.  It
      also does not requite any superuser privileges. See `PEP 405:
      Python Virtual Environments <https://peps.python.org/pep-0405/>`_ for more
      information.

      To create a virtual environment in the folder ``$HOME/myenv``,
      use the `venv <https://docs.python.org/3/library/venv.html>`_ module as follows.

      .. code-block:: bash

         # create virtual environment in folder $HOME/myenv
         python3 -m venv $HOME/myenv

      To activate the virtual environment type:

      .. code-block:: bash

         source $HOME/myenv/bin/activate

      This has to be done every time you log in or open a new terminal
      window and after you turn off the virtual environment with the
      ``deactivate`` command.

      When using CMake to build LAMMPS, you need to set
      ``CMAKE_INSTALL_PREFIX`` to the value of the ``$VIRTUAL_ENV``
      environment variable during the configuration step. For the
      traditional make procedure, no additional steps are needed.
      After compiling LAMMPS you can do a "Python package only"
      installation with ``make install-python`` and the LAMMPS Python
      package and the shared library file are installed into the
      following locations:

      +------------------------+--------------------------------------------------------+-------------------------------------------------------------+
      | File                   | Location                                               | Notes                                                       |
      +========================+========================================================+=============================================================+
      | LAMMPS Python Module   | * ``$VIRTUAL_ENV/lib/pythonX.Y/site-packages/lammps``  | ``X.Y`` depends on the installed Python version             |
      +------------------------+--------------------------------------------------------+-------------------------------------------------------------+
      | LAMMPS shared library  | * ``$VIRTUAL_ENV/lib/pythonX.Y/site-packages/lammps``  | ``X.Y`` depends on the installed Python version             |
      +------------------------+--------------------------------------------------------+-------------------------------------------------------------+

   .. tab:: In place usage

      You can also :doc:`compile LAMMPS <Build>` as usual in
      :ref:`"shared" mode <exe>` leave the shared library and Python
      package inside the source/compilation folders. Instead of
      copying the files where they can be found, you need to set the environment
      variables ``PYTHONPATH`` (for the Python package) and
      ``LD_LIBRARY_PATH`` (or ``DYLD_LIBRARY_PATH`` on macOS

      For Bourne shells (bash, ksh and similar) the commands are:

      .. code-block:: bash

         export PYTHONPATH=${PYTHONPATH}:${HOME}/lammps/python
         export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:${HOME}/lammps/src

      For the C-shells like csh or tcsh the commands are:

      .. code-block:: csh

         setenv PYTHONPATH ${PYTHONPATH}:${HOME}/lammps/python
         setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${HOME}/lammps/src

      On macOS you may also need to set ``DYLD_LIBRARY_PATH`` accordingly.
      You can make those changes permanent by editing your ``$HOME/.bashrc``
      or ``$HOME/.login`` files, respectively.

      .. note::

         The ``PYTHONPATH`` needs to point to the parent folder that contains the ``lammps`` package!

In case you run into an "externally-managed-environment" error when
trying to install the LAMMPS Python module, please refer to
:ref:`corresponding paragraph <externally_managed>` in the Python HOWTO
page to learn about options for handling this error.

To verify if LAMMPS can be successfully started from Python, start the
Python interpreter, load the ``lammps`` Python module and create a
LAMMPS instance.  This should not generate an error message and produce
output similar to the following:

   .. code-block:: console

      $ python
      Python 3.8.5 (default, Sep  5 2020, 10:50:12)
      [GCC 10.2.0] on linux
      Type "help", "copyright", "credits" or "license" for more information.
      >>> import lammps
      >>> lmp = lammps.lammps()
      LAMMPS (18 Sep 2020)
      using 1 OpenMP thread(s) per MPI task
      >>>

.. note::

   Unless you opted for "In place use", you will have to rerun the installation
   any time you recompile LAMMPS to ensure the latest Python package and shared
   library are installed and used.

.. note::

   If you want Python to be able to load different versions of the
   LAMMPS shared library with different settings, you will need to
   manually copy the files under different names
   (e.g. ``liblammps_mpi.so`` or ``liblammps_gpu.so``) into the
   appropriate folder as indicated above. You can then select the
   desired library through the *name* argument of the LAMMPS object
   constructor (see :ref:`python_create_lammps`).

.. _python_install_mpi4py:

Extending Python to run in parallel
===================================

If you wish to run LAMMPS in parallel from Python, you need to extend
your Python with an interface to MPI.  This also allows you to
make MPI calls directly from Python in your script, if you desire.

We have tested this with `MPI for Python <https://mpi4py.readthedocs.io/>`_
(aka mpi4py) and you will find installation instruction for it below.

Installation of mpi4py (version 4.0.1 as of Feb 2025) can be done as
follows:

- Via ``pip`` into a local user folder with:

  .. code-block:: bash

     python3 -m pip install --user mpi4py

- Via ``dnf`` into a system folder for RedHat/Fedora systems:

  .. code-block:: bash

     # for use with OpenMPI
     sudo dnf install python3-mpi4py-openmpi
     # for use with MPICH
     sudo dnf install python3-mpi4py-mpich

- Via ``pip`` into a virtual environment (see above):

  .. code-block:: console

     $ source $HOME/myenv/activate
     (myenv)$ python -m pip install mpi4py

- Via ``pip`` into a system folder (not recommended):

  .. code-block:: bash

     sudo python3 -m pip install mpi4py

For more detailed installation instructions and additional options,
please see the `mpi4py installation <https://mpi4py.readthedocs.io/en/stable/install.html>`_ page.


To use ``mpi4py`` and LAMMPS in parallel from Python, you **must** make
certain that **both** are using the **same** implementation and version
of MPI library.  If you only have one MPI library installed on your
system this is not an issue, but it can be if you have multiple MPI
installations (e.g. on an HPC cluster to be selected through environment
modules).  Your LAMMPS build is explicit about which MPI it is using,
since it is either detected during CMake configuration or in the
traditional make build system you specify the details in your low-level
``src/MAKE/Makefile.foo`` file. The installation process of ``mpi4py``
uses the ``mpicc`` command to find information about the MPI it uses to
build against.  And it tries to load "libmpi.so" from the
``LD_LIBRARY_PATH``.  This may or may not find the MPI library that
LAMMPS is using.  If you have problems running both mpi4py and LAMMPS
together, this is an issue you may need to address, e.g. by loading the
module for different MPI installation so that mpi4py finds the right
one.

If you have successfully installed mpi4py, you should be able to run
Python and type

.. code-block:: python

   from mpi4py import MPI

without error.  You should also be able to run Python in parallel
on a simple test script

.. code-block:: bash

   mpirun -np 4 python3 test.py

where ``test.py`` contains the lines

.. code-block:: python

   from mpi4py import MPI
   comm = MPI.COMM_WORLD
   print("Proc %d out of %d procs" % (comm.Get_rank(),comm.Get_size()))

and see one line of output for each processor you run on.  Please note
that the order of the lines is not deterministic

.. code-block:: console

   $ mpirun -np 4 python3 test.py
   Proc 0 out of 4 procs
   Proc 1 out of 4 procs
   Proc 2 out of 4 procs
   Proc 3 out of 4 procs
