5 CHAPTER TWO USING THE PYTHON INTERPRETER 2.1 Invoking the Interpreter 1 On Unix, the Python 3.x interpreter is by default not installed with the executable named python , so that it does not conflict with a simultaneously installed Python 2.x executable. All command line options are described in using-on-general. When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing$_{-i}$before the script. Some Python modules are also useful as scripts. These can be invoked using$_{python -m module [arg] ...}$ , which executes the source file for module as if you had spelled out its full name on the command line. A second way of starting the interpreter is$_{python -c command [arg] ...}$ , which executes the statement(s) in command , analogous to the shell’s$_{-c}$option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote command in its entirety with single quotes. The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a script from that file. The interpreter’s line-editing features include interactive editing, history substitution and code completion on systems that support the GNU Readline library. Perhaps the quickest check to see whether command line editing is supported is typing$_{Control-P}$ to the first Python prompt you get. If it beeps, you have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happen, or if ^P is echoed, command line editing isn’t available; you’ll only be able to use backspace to remove characters from the current line. Typing an end-of-file character ($_{Control-D}$ on Unix,$_{Control-Z}$on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t work, you can exit the interpreter by typing the following command:$_{quit()}$ . On Windows machines where you have installed Python from the Microsoft Store, the$_{python3.9}$ command will be available. If you have the py.exe launcher installed, you can use the$_{py}$ command. See setting-envvars for other ways to launch Python. to the shell. 1 Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g.,$_{/usr/local/python}$ is a popular alternative location.) The Python interpreter is usually installed as /usr/local/bin/python3.9 on those machines where it is available; putting$_{/usr/local/bin}$ in your Unix shell’s search path makes it possible to start it by typing the command: python3.9