# Calling Python from LAMMPS

LAMMPS has several commands which can be used to invoke Python code
directly from an input script:

-   [python](python)
-   [variable python](variable)
-   [fix python/invoke](fix_python_invoke)
-   [pair_style python](pair_python)

The [python](python) command which can be used to define and execute a
Python function that you write the code for. The Python function can
also be assigned to a LAMMPS python-style variable via the
[variable](variable) command. Each time the variable is evaluated,
either in the LAMMPS input script itself, or by another LAMMPS command
that uses the variable, this will trigger the Python function to be
invoked.

The Python code for the function can be included directly in the input
script or in an auxiliary file. The function can have arguments which
are mapped to LAMMPS variables (also defined in the input script) and it
can return a value to a LAMMPS variable. This is thus a mechanism for
your input script to pass information to a piece of Python code, ask
Python to execute the code, and return information to your input script.

Note that a Python function can be arbitrarily complex. It can import
other Python modules, instantiate Python classes, call other Python
functions, etc. The Python code that you provide can contain more code
than the single function. It can contain other functions or Python
classes, as well as global variables or other mechanisms for storing
state between calls from LAMMPS to the function.

The Python function you provide can consist of \"pure\" Python code that
only performs operations provided by standard Python. However, the
Python function can also \"call back\" to LAMMPS through its
Python-wrapped library interface, in the manner described in the [Python
run](Python_run) doc page. This means it can issue LAMMPS input script
commands or query and set internal LAMMPS state. As an example, this can
be useful in an input script to create a more complex loop with
branching logic, than can be created using the simple looping and
branching logic enabled by the [next](next) and [if](if) commands.

See the [python](python) page and the [variable](variable) doc page for
its python-style variables for more info, including examples of Python
code you can write for both pure Python operations and callbacks to
LAMMPS.