2. Using the Python Interpreter
2.1 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
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 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
-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.)
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2.1.1 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 -c
command 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.
2.1.2 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!
2.2 The Interpreter and Its Environment
2.2.1 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
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.
2.2.2 Executable Python Scripts
On BSD'ish Unix systems, Python scripts
can be made directly executable, like shell scripts, by putting the line
(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 a 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: iso-8859-1 -*-
With that declaration, all characters in the source file will be treated as iso-8859-1,
and it will be possible to directly write Unicode string literals in the selected
encoding. The list of possible encodings can be found in the Python Library Reference,
in the section on codecs.
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 #! files.
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.
2.2.4 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)
Footnotes
- ... prompt.2.1
- A problem with the GNU Readline package may prevent this.
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