# Development environment quickstart guide using `pip` on Ubuntu Linux

This is a high-level overview of what is needed to set up a development environment. This is only one possible way out of many. This guide assumes you have a fresh install of Ubuntu Linux 20.04, which only has a `python3` executable. We also assume you have already installed `git`, cloned the SciPy repository and initialized/updated git submodules.

## Installing the system-level dependencies

First, you will also need the compilers for C, C++ and Fortran:

    sudo apt install -y gcc g++ gfortran

SciPy also requires BLAS and LAPACK libraries. You can install several variants (ATLAS, OpenBLAS etc), but here we take the simplest option:

    sudo apt install -y liblapack-dev

## Installing the python-level dependencies

Start with installing `pip`:

    sudo apt install -y python3-pip

All further work should proceed in a virtual environment. Popular options include the standard library `venv` module or a separate `virtualenv` package. There are muliple third-party tutorials on how to set up a virtual environment, so we cover only briefly these two options here.

Note

We repeat: all work should happen in a virtual environment. Never use `sudo pip`.

### Using `virtualenv`

Install the `virtualenvwrapper` package:

    python3 -m pip install virtualenvwrapper --user

Edit the `.bashrc` file to add some environment variables which are used internally by the `virtualenvwrapper`:

    export WORKON_HOME=$HOME/virtualenvs
    export VIRTUALENVWRAPPER_PYTHON=/usr/bin/python3
    . $HOME/.local/bin/virtualenvwrapper.sh

Here we store the virtualenvs in a `virtualenvs` folder in the home directory. (you might need to create the folder manually).

Now open a new terminal window for the changes to the `.bashrc` to take effect.

Create a new virtual environment and activate it:

    mkvirtualenv scipy-dev

Your command prompt now lists the name of your new environment, like so `(scipy-dev)$`. This means that the environment is active. If it is not, activate it manually with:

    workon scipy-dev

Note `mkvirtualenv` and `workon` commands come from the `virtualwrapper` package.

### Using the standard-library `venv` package

Install the `venv` package:

    sudo apt install -y python3-venv

Change the directory to your home folder and create a directory `.venvs` there. Create the virtualenvironment:

    python3 -m venv scipy-dev

To activate the environment, use

    source $HOME/.venvs/scipy-dev/bin/activate

Your command prompt now lists the name of your new environment, like so `(scipy-dev)$`.

(For the official docs for the `venv` package see https://docs.python.org/3/tutorial/venv.html).

## Building SciPy

Inside the `scipy-dev` environment, install the python-level dependencies:

    python -m pip install numpy pytest cython pythran pybind11

Note that when the virtual environment is active, the system-wide names `pip3` and `python3` are aliased to `pip` and `python`, respectively.

Now that you have all needed dependencies, navigate to the directory where you cloned the source code into, and build SciPy (this takes a while):

    python setup.py build

Optionally, test it:

    python runtests.py