Setting Up Your Python Environment

Python in your browser

In SPASCLE you don't need to install anything. Python runs directly inside your web browser, on your own computer, using a version of Python built for browsers called Pyodide. Your code is not sent to a server.

Every example with a Try it hands-on button opens an editor. It has the following parts:

• The editor, where you type and change code. It highlights Python syntax in colour to make code easier to read.
• The Run button, which runs the whole program. The shortcut is Ctrl + Enter (or Cmd + Enter on a Mac).
• The output panel, which shows everything your program prints, and any error messages.
• Toolbar buttons to switch the editor between dark and light, copy the code, download it as a .py file, and Reset the original example.

The first time you press Run, Python itself has to load, which takes a few seconds. After that, programs run almost instantly. Try it with this program, which prints the version of Python that is running.

import sysprint("Python version:", sys.version.split()[0])

import sys loads sys, a module that comes with Python and gives information about the Python system. You will learn how modules and import work later in the course.

Scripts and the interactive prompt

A Python program saved in a file is called a script. Script files end in .py, for example density.py. When you run a script, Python reads the whole file from top to bottom and carries out each statement. The editors in this course work the same way.

In a script, Python only shows something if you ask it to with print(). A calculation on its own is worked out, but the result is not displayed. This surprises many beginners, so try it:

1085 + 273.15          # calculated, but not shownprint(1085 + 273.15)   # calculated and shown

Once Python is installed on your computer, you can also use the interactive prompt (also called the REPL). You type one statement at a time after the >>> symbol and see the result immediately, without print(). It is handy for quick calculations and for testing small ideas. For anything you want to keep, write a script.

Installing Python on your own computer

Everything in this course works in the browser, but for your own research you will want Python installed locally. These are the common options:

To run a script from a terminal (Command Prompt on Windows), move to the folder that contains it and type python density.py. On some systems the command is python3.

Packages

Python comes with a large standard library, but much of its scientific power comes from packages: collections of code written by others that you install separately. NumPy and Matplotlib are packages. On your own computer you install them with a package manager, for example pip install numpy or conda install numpy. In SPASCLE, the packages used in the course are loaded automatically when your code needs them.