System Command Lines and Files

Although the interactive prompt is great for experimenting and testing, it has one big disadvantage: programs you type there go away as soon as the Python interpreter executes them. Because the code you type interactively is never stored in a file, you can’t run it again without retyping it from scratch. Cut-and-paste and command recall can help some here, but not much, especially when you start writing larger programs. To cut and paste code from an interactive session, you would have to edit out Python prompts, program outputs, and so on—not exactly a modern software development methodology!

To save programs permanently, you need to write your code in files, which are usually known as modules. Modules are simply text files containing Python statements. Once coded, you can ask the Python interpreter to execute the statements in such a file any number of times, and in a variety of ways—by system command lines, by file icon clicks, by options in the IDLE user interface, and more. Regardless of how it is run, Python executes all the code in a module file from top to bottom each time you run the file.

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Terminology in this domain can vary somewhat. For instance, module files are often referred to as programs in Python—that is, a program is considered to be a series of precoded statements stored in a file for repeated execution. Module files that are run directly are also sometimes called scripts—an informal term usually meaning a top-level program file. Some reserve the term “module” for a file imported from another file. (More on the meaning of “top-level” and imports in a few moments.)

Whatever you call them, the next few sections explore ways to run code typed into module files. In this section, you’ll learn how to run files in the most basic way: by listing their names in a python command line entered at your computer’s system prompt. Though it might seem primitive to some, for many programmers a system shell command-line window, together with a text editor window, constitutes as much of an integrated development environment as they will ever need.