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<li class="toctree-l1"><a class="reference internal" href="Install.html">2. Install LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Build.html">3. Build LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Run_head.html">4. Run LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Library.html">1. LAMMPS Library Interfaces</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="Python_head.html">2. Use Python with LAMMPS</a><ul class="current">
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<li class="toctree-l2"><a class="reference internal" href="Python_overview.html">2.1. Overview</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_install.html">2.2. Installation</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_run.html">2.3. Run LAMMPS from Python</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_module.html">2.4. The <code class="docutils literal notranslate"><span class="pre">lammps</span></code> Python module</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_ext.html">2.5. Extending the Python interface</a></li>
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<li class="toctree-l2 current"><a class="current reference internal" href="#">2.6. Calling Python from LAMMPS</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_formats.html">2.7. Output Readers</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_examples.html">2.8. Example Python scripts</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_error.html">2.9. Handling LAMMPS errors</a></li>
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<li class="toctree-l2"><a class="reference internal" href="Python_trouble.html">2.10. Troubleshooting</a></li>
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</ul>
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</li>
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<li class="toctree-l1"><a class="reference internal" href="Modify.html">3. Modifying & extending LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Developer.html">4. Information for Developers</a></li>
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<li class="breadcrumb-item"><a href="Python_head.html"><span class="section-number">2. </span>Use Python with LAMMPS</a></li>
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<li class="breadcrumb-item active"><span class="section-number">2.6. </span>Calling Python from LAMMPS</li>
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<a href="https://www.lammps.org"><img src="_static/lammps-logo.png" width="64" height="16" alt="LAMMPS Homepage"></a> | <a href="Commands_all.html">Commands</a>
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<p><span class="math notranslate nohighlight">\(\renewcommand{\AA}{\text{Å}}\)</span></p>
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<section id="calling-python-from-lammps">
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<h1><span class="section-number">2.6. </span>Calling Python from LAMMPS<a class="headerlink" href="#calling-python-from-lammps" title="Link to this heading"></a></h1>
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<p>LAMMPS has several commands which can be used to invoke Python
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code directly from an input script:</p>
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<ul class="simple">
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<li><p><a class="reference internal" href="python.html"><span class="doc">python</span></a></p></li>
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<li><p><a class="reference internal" href="variable.html"><span class="doc">variable python</span></a></p></li>
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<li><p><a class="reference internal" href="fix_python_invoke.html"><span class="doc">fix python/invoke</span></a></p></li>
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<li><p><a class="reference internal" href="pair_python.html"><span class="doc">pair_style python</span></a></p></li>
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</ul>
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<p>The <a class="reference internal" href="python.html"><span class="doc">python</span></a> command which can be used to define and
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execute a Python function that you write the code for. The Python
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function can also be assigned to a LAMMPS python-style variable via
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the <a class="reference internal" href="variable.html"><span class="doc">variable</span></a> command. Each time the variable is
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evaluated, either in the LAMMPS input script itself, or by another
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LAMMPS command that uses the variable, this will trigger the Python
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function to be invoked.</p>
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<p>The Python code for the function can be included directly in the input
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script or in an auxiliary file. The function can have arguments which
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are mapped to LAMMPS variables (also defined in the input script) and
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it can return a value to a LAMMPS variable. This is thus a mechanism
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for your input script to pass information to a piece of Python code,
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ask Python to execute the code, and return information to your input
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script.</p>
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<p>Note that a Python function can be arbitrarily complex. It can import
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other Python modules, instantiate Python classes, call other Python
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functions, etc. The Python code that you provide can contain more
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code than the single function. It can contain other functions or
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Python classes, as well as global variables or other mechanisms for
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storing state between calls from LAMMPS to the function.</p>
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<p>The Python function you provide can consist of “pure” Python code that
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only performs operations provided by standard Python. However, the
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Python function can also “call back” to LAMMPS through its
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Python-wrapped library interface, in the manner described in the
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<a class="reference internal" href="Python_run.html"><span class="doc">Python run</span></a> doc page. This means it can issue LAMMPS
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input script commands or query and set internal LAMMPS state. As an
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example, this can be useful in an input script to create a more
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complex loop with branching logic, than can be created using the
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simple looping and branching logic enabled by the <a class="reference internal" href="next.html"><span class="doc">next</span></a> and
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<a class="reference internal" href="if.html"><span class="doc">if</span></a> commands.</p>
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<p>See the <a class="reference internal" href="python.html"><span class="doc">python</span></a> page and the <a class="reference internal" href="variable.html"><span class="doc">variable</span></a>
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doc page for its python-style variables for more info, including
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examples of Python code you can write for both pure Python operations
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and callbacks to LAMMPS.</p>
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</section>
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