Running Python

What a terminal is

A terminal is a text window where you type commands to your computer instead of clicking. Every operating system ships one. You run the builds from here.

Folders and the current directory

The terminal is always sitting inside one folder, called the current working directory. Commands act relative to it, so where you are matters.

Change directory

cd path/to/your/folder

Use this to move into the folder that holds your script before you run it.

Run a script

Save your code as a file, for example q_learning.py, then run:

python3 q_learning.py

REPL versus script

Typing python3 on its own opens an interactive prompt, the REPL, where you run one line at a time. Running python3 file.py runs a whole file start to finish. The builds are files, so use the file form.

Stop a stuck script

Press Ctrl-C in the terminal to interrupt a running script.

Common traps

• Running from the wrong folder. A "file not found" usually means you are not in the folder that holds the script. cd into it first.
• Typing in the wrong place. Terminal commands go in the terminal; Python code goes in the REPL or a file. If cd errors with a syntax message, you are probably inside the Python REPL. Type exit() to leave it.
• Hidden file extensions. Some systems hide extensions, so an editor may save q_learning.py.txt. Turn on "show file extensions" and check the real name.