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Python Tutorial |  |
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Python Tutorial | |
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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 EOF 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 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 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 EOF 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.)
When known to the interpreter, the script name and additional
arguments thereafter are passed to the script in the variable
sys.argv, which is a list of strings. Its length is at least
one; when no script and no arguments are given, sys.argv[0] is
an empty string. When the script name is given as '-' (meaning
standard input), sys.argv[0] is set to '-'. When the
-c command form is used, sys.argv[0] is set to
'-c'. Options found after -c command are not consumed
by the Python interpreter's option processing but left in
sys.argv for the command to handle.
When commands are read from a tty, the interpreter is said to be in interactive mode. In this mode it prompts for the next command with the primary prompt, usually three greater-than signs (">>> "); for continuation lines it prompts with the secondary prompt, by default three dots ("... "). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt, e.g.:
python Python 1.5.2b2 (#1, Feb 28 1999, 00:02:06) [GCC 2.8.1] on sunos5 Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam >>>
Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:
>>> the_world_is_flat = 1 >>> if the_world_is_flat: ... print "Be careful not to fall off!" ... Be careful not to fall off!
When an error occurs, the interpreter prints an error
message and a stack trace. In interactive mode, it then returns to
the primary prompt; when input came from a file, it exits with a
nonzero exit status after printing
the stack trace. (Exceptions handled by an except clause in a
try statement are not errors in this context.) Some errors are
unconditionally fatal and cause an exit with a nonzero exit; this
applies to internal inconsistencies and some cases of running out of
memory. All error messages are written to the standard error stream;
normal output from the executed commands is written to standard
output.
Typing the interrupt character (usually Control-C or DEL) to the
primary or secondary prompt cancels the input and returns to the
primary prompt.2.1Typing an interrupt while a command is executing raises the
KeyboardInterrupt exception, which may be handled by a
try statement.
On BSD'ish Unix systems, Python scripts can be made directly executable, like shell scripts, by putting the line
#! /usr/bin/env python
(assuming that the interpreter is on the user's $PATH) at the beginning of the script and giving the file an executable mode. The "#!" must be the first two characters of the file. Note that the hash, or pound, character, "#", is used to start a comment in Python.
When you use Python interactively, it is frequently handy to have some standard commands executed every time the interpreter is started. You can do this by setting an environment variable named $PYTHONSTARTUP to the name of a file containing your start-up commands. This is similar to the .profile feature of the Unix shells.
This file is only read in interactive sessions, not when Python reads
commands from a script, and not when /dev/tty is given as the
explicit source of commands (which otherwise behaves like an
interactive session). It is executed in the same name space where
interactive commands are executed, so that objects that it defines or
imports can be used without qualification in the interactive session.
You can also change the prompts sys.ps1 and sys.ps2 in
this file.
If you want to read an additional start-up file from the current directory, you can program this in the global start-up file, e.g. "execfile('.pythonrc.py')". If you want to use the startup file in a script, you must do this explicitly in the script:
import os
if os.environ.get('PYTHONSTARTUP') \
and os.path.isfile(os.environ['PYTHONSTARTUP']):
execfile(os.environ['PYTHONSTARTUP'])
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Python Tutorial | |
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