Create Virtual_Environments.md

Author: ashki23

Virtual_Environments.md

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+# Virtual Environments in Python
+[The Python Tutorial](https://docs.python.org/3/tutorial/venv.html)
+
+Python applications will often use packages and modules that don’t come as part of the standard library. Applications will sometimes need a specific version of a library, because the application may require that a particular bug has been fixed or the application may be written using an obsolete version of the library’s interface.
+
+This means it may not be possible for one Python installation to meet the requirements of every application. If application A needs version 1.0 of a particular module but application B needs version 2.0, then the requirements are in conflict and installing either version 1.0 or 2.0 will leave one application unable to run.
+
+The solution for this problem is to create a virtual environment, a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.
+
+## venv
+To create a virtual environment, decide upon a directory where you want to place it, and run the venv module as a script with the directory path:
+```bash
+python3 -m venv tutorial-env
+```
+
+This will create the `tutorial-env` directory if it doesn’t exist, and also create directories inside it containing a copy of the Python interpreter, the standard library, and various supporting files. Once you’ve created a virtual environment, you may activate it by:
+```bash
+source tutorial-env/bin/activate
+```
+
+We can use the following Bash scripts to create `tutorial-env` if it does not exist and activate it and install modules:
+```bash
+#!/bin/bash
+
+if [ ! -d tutorial-env ]; then
+  python3 -m venv tutorial-env
+fi
+
+source tutorial-env/bin/activate
+
+pip install --upgrade pip
+pip install redis
+pip install gitpython
+```
+
+As a shortkey, you might use `.` instead `source`. To exit from a virtual environment run `deactivate`. 
+
+## Miniconda
+To use use Conda as an environment management system, you need to install Miniconda (or Anaconda) and use `conda create` to make a new environment by:
+```bash
+conda create --name <env_name> <pkg name> <pkg name> ...
+```
+
+You also can use `--yes` flag to set up the environment without a question and use `--prefix <env_path>` to setup the environment in the path that you want (note that you can not use `--prefix` and `--name` at the same time). To see list of environments, use the following:
+```bash 
+conda env list
+```
+
+You might also use `conda info --envs` to see list of envs. To activate/deactivate an environment use the following:
+```bash
+conda activate <env_name or env_path>
+conda deactivate
+```
+
+Note that, for conda versions prior to 4.6 you need to use `source` instead of `conda` in the above commands. Now, you can use the following to update `conda` and install new libraries and software:
+```bash
+conda update conda
+conda install <pkg name>
+```
+
+To remove a package you can use:
+```bash 
+conda remove <pkg name>
+```
+
+And to remove cache files use:
+```bash
+conda clean --all
+```
+
+**Note**: before removing packages or caches, make sure you are in the right virtual environment. You can use `conda info` and `conda env list` to find your current environment and use `source activate <env name or path>` or `source deactivate` to go to the rigth env.
+
+To remove a virtual environment you can use:
+```bash
+conda env remove --name <env_name> or --prefix <env_path>
+```
+**Note**: you cannot remove an environment when it is activate. Note that sometime you should `deactivate` multiple times, if you `activate` more than once.
+
+In a **HPC** cluster system, first you need to load `miniconda3` module to be able to use `conda`. You can use the following as a template to build your new environment:
+```bash
+module load miniconda3
+
+if [ ! -d yourlocal_env ]; then
+conda create --yes --prefix ./yourlocal_env <pkg name> <pkg name> ...
+fi
+source activate ./yourlocal_env
+```