The Interactive Prompt

Perhaps the simplest way to run Python programs is to type them at Python’s interactive command line, sometimes called the interactive prompt. There are a variety of ways to start this command line: in an IDE, from a system console, and so on. Assuming the interpreter is installed as an executable program on your system, the most platform-neutral way to start an interactive interpreter session is usually just to type python at your operating system’s prompt, without any arguments. For example:

% python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ...
Type "help", "copyright", "credits" or "license" for more information.
>>>

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Typing the word “python” at your system shell prompt like this begins an interactive Python session; the “%” character at the start of this listing stands for a generic system prompt in this book—it’s not input that you type yourself. The notion of a system shell prompt is generic, but exactly how you access it varies by platform:

 

 
  • On Windows, you can type python in a DOS console window (a.k.a. the Command Prompt, usually found in the Accessories section of the Start→Programs menu) or in the Start→Run... dialog box.
  • On Unix, Linux, and Mac OS X, you might type this command in a shell or terminal window (e.g., in an xterm or console running a shell such as ksh or csh).
  • Other systems may use similar or platform-specific devices. On handheld devices, for example, you generally click the Python icon in the home or application window to launch an interactive session.

If you have not set your shell’s PATH environment variable to include Python’s install directory, you may need to replace the word “python” with the full path to the Python executable on your machine. On Unix, Linux, and similar, /usr/local/bin/python or /usr/bin/python will often suffice. On Windows, try typing C:\Python30\python (for version 3.0):

C:\misc> c:\python30\python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ...
Type "help", "copyright", "credits" or "license" for more information.
>>>

Alternatively, you can run a change-directory command to go to Python’s install directory before typing “python”—try the cd c:\python30 command on Windows, for example:

C:\misc> cd C:\Python30
C:\Python30> python
Python 3.0.1 (r301:69561, Feb 13 2009, 20:04:18) [MSC v.1500 32 bit (Intel)] ...
Type "help", "copyright", "credits" or "license" for more information.
>>>

On Windows, besides typing python in a shell window, you can also begin similar interactive sessions by starting IDLE’s main window (discussed later) or by selecting the “Python (command line)” menu option from the Start button menu for Python, as shown in Figure 2-1 back in Chapter 2. Both spawn a Python interactive prompt with equivalent functionality; typing a shell command isn’t necessary.