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.)
On Windows machines, the Python installation is usually placed in
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:
set path=%path%;C:\python25
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 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 second way of starting the interpreter is "python-ccommand [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 "python-mmodule [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 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
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 -ccommand is used, sys.argv[0] is set to '-c'.
When -mmodule is used, sys.argv[0]
is set to the full name of the located module. Options found after
-ccommand or -mmodule are not consumed by the Python interpreter's option processing
but left in sys.argv for the command or module to handle.
Interactive Mode
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:
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!
The Interpreter and Its Environment
Error Handling
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
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. Typing
an interrupt while a command is executing raises the
KeyboardInterrupt exception, which may be handled by a
try statement.
Executable Python Scripts
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. On some platforms, this first line
must end with a Unix-style line ending ("\n"),
not a Mac OS ("\r") or Windows ("\r\n")
line ending. Note that the hash, or pound, character, "#",
is used to start a comment in Python.
The script can be given an executable mode, or permission, using the
chmod command:
It is possible to use encodings different than ASCII in Python source files.
The best way to do it is to put one more special comment line right after the
#! line to define the source file encoding:
# -*- coding: encoding -*-
With that declaration, all characters in the source file will be treated as
having the encoding encoding, and it will be possible to directly
write Unicode string literals in the selected encoding.
For example, to write Unicode literals including the Euro currency symbol,
the ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal
value 164. This script will print the value 8364 (the Unicode codepoint
corresponding to the Euro symbol) and then exit:
If your editor supports saving files as UTF-8 with a UTF-8
byte order mark (aka BOM), you can use that instead of an encoding
declaration. IDLE supports this capability if Options/General/Default
Source Encoding/UTF-8 is set. Notice that this signature is not
understood in older Python releases (2.2 and earlier), and also not understood
by the operating system for script files with #! lines (only used
on Unix systems).
By using UTF-8 (either through the signature or an encoding declaration),
characters of most languages in the world can be used simultaneously in string
literals and comments. Using non-ASCII characters in identifiers is not
supported. To display all these characters properly, your editor must recognize
that the file is UTF-8, and it must use a font that supports all the characters
in the file.
The Interactive Startup File
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 namespace 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 using code like "if
os.path.isfile('.pythonrc.py'): execfile('.pythonrc.py')". If you want to
use the startup file in a script, you must do this explicitly in the script:
import os
filename = os.environ.get('PYTHONSTARTUP')
if filename and os.path.isfile(filename):
execfile(filename)
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