9 Installing Python and the opensafely command line interface

Environment variables in computer operating systems contain important strings of text
The PATH environment variable is a list of folders which the computer searches in when you type the name of an executable into the command line shell program (usually zsh on macOS, bash on Ubuntu, cmd or Powershell on Windows)
To use the python/python3 and pip/pip3 commands at the shell command line we need to install Python and make sure the folder containing its executable is in our PATH environment variable (unless you already know all of this and are going to run Python in Anaconda through the Anaconda Prompt)

9.1 macOS

If you have a Mac, the macOS operating system comes with an old-ish version of Python 2.7
I recommend installing Python 3 through homebrew
```
brew install python
```
When you open Terminal
- See the contents of PATH with echo $PATH (note use ${PATH} in shell scripts)
- you should be able to find the python/python3 exectables with the which command

You have a number of choices where to install Python from
- Microsoft store, .e.g. Python 3.10 from here
- Python installer from here
- Anaconda installer from here
Despite not being recommend - it is better for you to add Anaconda/Python to your PATH in the installer options, i.e., check the first box on this screen
- And in the Python installer check the box adding Python to PATH
Open Windows Terminal
- you can see the contents of PATH in Powershell with $Env:Path
- and in cmd with echo %PATH%
- you can see the location of the Python executable in cmd with where python/where python3
If you installed Anaconda and you did not add its folders to PATH then you need to install and run opensafely using the Anaconda prompt - you find this as a program under the Start menu

As long as the python/python3 and pip/pip3 executables are now on your PATH you can simply run in your shell program
```
pip install opensafely   
```
This will additionally install its dependency package the cohortextractor package into your Python installation and you should now be able to run opensafely commands such as
```
opensafely run run_all
```