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(python2.1-tut.info)Invoking the Interpreter

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Invoking the Interpreter
========================

The Python interpreter is usually installed as `/usr/local/bin/python'
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

     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 Note: 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.)

Argument Passing

Interactive Mode