====== Python ======

In this guide, I expect you are using Python3 (and ''pip3''), but (for old projects) you can use Python2 (and ''pip2'') as well. 

  * You can use the system Python
  * You can use any Python from ''/opt/python/'', e.g. for 3.11.0 add the following line to your .bashrc: <code>export PATH=/opt/python/3.11.0/bin:"${PATH}"</code>
  * You can install modules with ''pip''. It is recommended to use [[#virtual environments]] for installing new packages. If you are not using virtual environments, you must add the ''-''''-user'' option, so it is installed in your home. In this case, don't forget the executable scripts are installed in ''~/.local/bin/'', so add the following line to your .bashrc: <code>export PATH=~/.local/bin:"${PATH}"</code>
  * Both the system Python3 and the Python3 from ''/opt/python/'' come with an old version of pip3, so it is recommended to update it with <code>pip3 install --user --upgrade pip</code>
  * There are no UFAL-wide pre-installed Python modules. You need to install all modules you need yourself (with pip3).

===== Virtual environments =====
You can use [[https://docs.python.org/3/tutorial/venv.html|virtual environments]], e.g. one for each project (or a group of projects), so the version requirements of different projects do not collide.

Create a new virtual environment with:
    python3 -m venv my-project-name
or (for a specific version of python):
    /opt/python/$PYTHON_VERSION/bin/python3 -m venv my-project-name

where ''my-project-name'' can be a relative or absolute path to a directory which will be created.
**Caveat1:** the path to the directory should be rather short, there is a limit on shebang paths so you can get strange errors like ''ipython3: bad interpreter: No such file or directory'' if your ipython3 is installed in the venv starts with ''#!/..a..very..long..path..''
**Caveat2:** If ''my-project-name'' is a relative path, it will be concatenated with ''pwd -P'', i.e. with your actual directory with all symlinks resolved. If you want symlinks in the path, you must provide absolute path as ''my-project-name''. Note that e.g. ''/net/work/people'' is currently (2018-06) resolved by ''pwd -P'' to ''/lnet/.nfs/spec/work/people/'', but this path does not work on machines with native Lustre support.)

You can use any Python version for the new environment, e.g. ''/opt/python/3.5.4/bin/python3 -m venv my-project-name-python3.5''
Then activate it with
    source my-project-name/bin/activate
Now your prompt changes and you can install any modules (with pip3, but you can omit ''-''''-user''), e.g.
    pip3 install --upgrade pip tensorflow
If you run ''python'', it will be the version used when creating venv. Check it with
    which python
    python -V
Once finished, exit the virtual environment with
    deactivate
If you decide you don't need the environment and any modules installed in this environment, simply delete the ''my-project-name'' directory.

Note that there are many alternative ways how to create virtual environments and install modules. The ''venv'' module is available only for Python 3.3 and newer. For older versions you can use the third-party ''virtualenv'' tool:
   pip3 install --user --upgrade pip
   pip3 install --user virtualenv
   virtualenv my-project-name
   source my-project-name/bin/activate
   
==== IPython in virtual envs. ====
When using virtuaenvs, ipython is (in some versions) confused and uses an incorrect python interpreter. The following alias fixes this issue:
<code>
alias ipython="python -c 'import IPython; IPython.terminal.ipapp.launch_new_instance()'"
</code>

===== Installing directly from Git =====
If you are developing (contributing to) a Python tool version in git and want to install the newest version, you can use ''pip3 git+https:/''''/github.com/...'', provided the project has proper ''setup.py'' in the root directory.
You can also ''git clone'' the project manually and add the path to your ''$PYTHONPATH''. This way whenever you ''git pull'', you should make sure there are no new dependencies, e.g. with ''pip3 install --user -r requirements.txt'' if the project provides the ''requirements.txt'' file.
See e.g. [[https://github.com/udapi/udapi-python#install-udapi-for-developers|Udapi]] or [[https://github.com/ufal/neuralmonkey|Neural Monkey]] for examples of both ways.