Working with Python

Writing a Python program is only half the job — something must read those statements and make the computer carry them out. That something is the Python interpreter. To write and run (execute) a Python program, a Python interpreter must be installed on the computer; alternatively, an online Python interpreter can be used through the internet, with nothing installed locally. Either way, the interpreter is the engine that takes each Python statement and executes it.

The interpreter is also called the Python shell. When it is opened, it presents a screen of its own (a sample appears in Figure 3.1) whose most important element is a small symbol:

• >>> — this is the Python prompt. Its presence means the interpreter is ready to receive instructions. You type a command or statement at this prompt, and the interpreter executes it.

The prompt is worth internalising as a signal: whenever >>> is showing, the shell is waiting for your next instruction; while a statement is being executed, it is not.

Working directly at the prompt has one serious limitation: statements typed there are not saved for future use. Once the session is over, everything typed is gone, and running the same statements again means retyping them from scratch.

Saving work — script mode

The remedy is script mode: instead of typing statements live at the prompt, the whole program is written in a file, saved, and then the interpreter is asked to execute the program from that file. Such program files carry the .py extension and are also known as scripts. Beginners usually start in the interactive shell, but for any program longer than a few lines the code should always be saved in a file — that way it can be run again, corrected and extended at any time.

A Python script can be created in any editor. Python itself ships with a built-in editor called IDLE — short for Integrated Development and Learning Environment. Creating a program in IDLE follows a simple routine:

1. Open IDLE.
2. Choose File > New File to open a fresh editor window.
3. Type the program into that window.
4. Save it with a name of your choice. By default, Python scripts are saved in the Python installation folder.

Figure 3.3 shows a small program, prog3-1.py, written in the IDLE editor this way.

Running a script

Executing a saved program in script mode takes three steps:

1. Open the program in an editor — for example, open prog3-1.py in IDLE (as in Figure 3.3). …