Python Env
=========================

What is Python?
---------------

Python is a programming language that lets you work quickly and integrate
systems more effectively.

What Python version/environment can users find/set up on ENEAGRID/CRESCO?
-------------------------------------------------------------------------

On HPC clusters the default system Python is already available:

        =============== ==========================
        **System**      **Python version**         
        --------------- --------------------------
        CRESCO8            3.9.18
        --------------- -------------------------- 
        CRESCO7            3.9.16                
        --------------- -------------------------- 
        XCRESCO            3.9.16                    
        --------------- -------------------------- 
        CRESCO6            2.7.5                     
        =============== ========================== 


To check the Python version use the command

.. code-block:: bash

   python -V

.. note::

   If the user needs a different version of Python, you can load the :doc:`Scipy <scipy>` module 
   can be loaded by using the ``module`` command.

How to create a local Python environment by virtualenv tool
-----------------------------------------------------------

If the user needs to create a local environment where it is possible to install all Python modules required for a particular software, the ``virtualenv`` tool (already installed on CRESCO) can be useful.

Using ``virtualenv``, the user can generate and configure a local Python environment, also specifying the destination directory, such as a subfolder of the user home, in which all the necessary modules/packages can be installed.

.. note::

   The CRESCO6 user does not have to install ``virtualenv`` as described in many tutorials, because it is already available. It is only necessary to load the ``scipy`` or ``scipy39`` modules.

For example:

Create a subfolder in the user home, move into it and generate a local virtual environment named ``myenv`` by using ``virtualenv``:

.. code-block:: bash

   mkdir myLocalPython
   cd myLocalPython/
   python3 -m venv myenv

Check that the environment has been created:

.. code-block:: bash

   >ls
   myenv

Activate the local environment:

.. code-block:: bash

   source myenv/bin/activate

The ``myenv`` label in parentheses, before the username and hostname, indicates that the environment is active.

Then use local ``python`` and ``pip`` to install packages. For example, install the ``pandas`` package through a ``requirements.txt`` file:

.. code-block:: bash

   >which python
   ~/myLocalPython/myenv/bin/python

   >which pip
   ~/myLocalPython/myenv/bin/pip

   >echo "pandas" > requirements.txt
   >ls
   >myenv requirements.txt

   >cat requirements.txt
   pandas

   >pip install -r requirements.txt
   
   Collecting pandas
   Downloading pandas-1.1.3-cp36-cp36m-manylinux1_x86_64.whl (9.5 MB)
   ...
   Successfully installed numpy-1.19.2 pandas-1.1.3 python-dateutil-2.8.1 pytz-2020.1 six-1.15.0

To deactivate the virtual environment, use:

.. code-block:: bash

   deactivate

The ``myenv`` label in parentheses disappears when the environment is no longer active.

Links
~~~~~~

- `Python official website <https://www.python.org>`_
- `Installing packages and virtual environments <https://packaging.python.org/guides/installing-using-pip-and-virtual-environments/>`_
- `Virtualenv guide <https://www.enappi.com/python/virtualenv-guida/>`_