|An Introduction to Python|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
RRP £12.95 ($19.95)
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2.1 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
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.)
On Windows machines, the Python installation is usually placed in a directory like ‘C:\Python25’, though you can change this when you're running the installer. To add this directory to your path, you can type the following command into the command prompt in a DOS box:
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 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 A 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
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 second way of starting the interpreter is
-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.
Some Python modules are also useful as scripts. These can be invoked using
-m module [arg] ...’, which
executes the source file for module as if you had spelled out its
full name on the command line.
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
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
-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.)
|ISBN 0954161769||An Introduction to Python||See the print edition|