4.1 Invoking the Interpreter

The Python interpreter is usually installed as ‘/usr/bin/python’ or ‘/usr/local/bin/python’ on those machines where it is available; putting the appropriate directory in your UNIX shell's search path makes it possible to start it by typing the command

    python

to the shell. 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.)

Typing an end-of-file character (Control-D on UNIX, Control-Z on DOS or 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 commands: ‘import sys; sys.exit()’.

The interpreter's line-editing features usually aren't very sophisticated. On UNIX, whoever installed the interpreter may have enabled support for the GNU readline library, which adds more elaborate interactive editing and history features. 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 section A 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.

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.

A third 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 best to quote command in its entirety with double quotes.

Note that there is a difference between ‘python file’ and ‘python <file’. In the latter case, input requests from the program, such as calls to input() and raw_input(), are satisfied from file. Since this file has already been read until the end by the parser before the program starts executing, the program will encounter end-of-file immediately. In the former case (which is usually what you want) they are satisfied from whatever file or device is connected to standard input of the Python interpreter.

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. (This does not work if the script is read from standard input, for the same reason as explained in the previous paragraph.)