248 lines
10 KiB
ReStructuredText
248 lines
10 KiB
ReStructuredText
.. _tut-using:
|
|
|
|
****************************
|
|
Using the Python Interpreter
|
|
****************************
|
|
|
|
|
|
.. _tut-invoking:
|
|
|
|
Invoking the Interpreter
|
|
========================
|
|
|
|
The Python interpreter is usually installed as :file:`/usr/local/bin/python` on
|
|
those machines where it is available; putting :file:`/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., :file:`/usr/local/python` is a popular
|
|
alternative location.)
|
|
|
|
On Windows machines, the Python installation is usually placed in
|
|
:file:`C:\Python30`, 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:\python30
|
|
|
|
Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`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 :ref:`tut-interacting` 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
|
|
:option:`-c` option. Since Python statements often contain spaces or other
|
|
characters that are special to the shell, it is usually advised to quote
|
|
*command* in its entirety with single quotes.
|
|
|
|
Some Python modules are also useful as scripts. These can be invoked using
|
|
``python -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 calling
|
|
``sys.stdin.read()``, 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 :option:`-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.)
|
|
|
|
|
|
.. _tut-argpassing:
|
|
|
|
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
|
|
:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When
|
|
:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the
|
|
located module. Options found after :option:`-c` *command* or :option:`-m`
|
|
*module* are not consumed by the Python interpreter's option processing but
|
|
left in ``sys.argv`` for the command or module to handle.
|
|
|
|
|
|
.. _tut-interactive:
|
|
|
|
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 3.0a1 (py3k, Sep 12 2007, 12:21:02)
|
|
[GCC 3.4.6 20060404 (Red Hat 3.4.6-8)] on linux2
|
|
Type "help", "copyright", "credits" or "license" for more information.
|
|
>>>
|
|
|
|
.. XXX update for final release of Python 3.0
|
|
|
|
Continuation lines are needed when entering a multi-line construct. As an
|
|
example, take a look at this :keyword:`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!
|
|
|
|
|
|
.. _tut-interp:
|
|
|
|
The Interpreter and Its Environment
|
|
===================================
|
|
|
|
|
|
.. _tut-error:
|
|
|
|
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 :keyword:`except` clause in a :keyword:`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
|
|
:exc:`KeyboardInterrupt` exception, which may be handled by a :keyword:`try`
|
|
statement.
|
|
|
|
|
|
.. _tut-scripts:
|
|
|
|
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 :envvar:`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
|
|
:program:`chmod` command::
|
|
|
|
$ chmod +x myscript.py
|
|
|
|
On Windows systems, there is no notion of an "executable mode". The Python
|
|
installer automatically associates ``.py`` files with ``python.exe`` so that
|
|
a double-click on a Python file will run it as a script. The extension can
|
|
also be ``.pyw``, in that case, the console window that normally appears is
|
|
suppressed.
|
|
|
|
|
|
Source Code Encoding
|
|
--------------------
|
|
|
|
By default, Python source files are treated as encoded in UTF-8. In that
|
|
encoding, characters of most languages in the world can be used simultaneously
|
|
in string literals, identifiers and comments --- although the standard library
|
|
only uses ASCII characters for identifiers, a convention that any portable code
|
|
should follow. 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.
|
|
|
|
It is also possible to specify a different encoding for source files. In order
|
|
to do this, put one more special comment line right after the ``#!`` line to
|
|
define the source file encoding::
|
|
|
|
# -*- coding: encoding -*-
|
|
|
|
With that declaration, everything in the source file will be treated as having
|
|
the encoding *encoding* instead of UTF-8. The list of possible encodings can be
|
|
found in the Python Library Reference, in the section on :mod:`codecs`.
|
|
|
|
For example, if your editor of choice does not support UTF-8 encoded files and
|
|
insists on using some other encoding, say Windows-1252, you can write::
|
|
|
|
# -*- coding: cp-1252 -*-
|
|
|
|
and still use all characters in the Windows-1252 character set in the source
|
|
files. The special encoding comment must be in the *first or second* line
|
|
within the file.
|
|
|
|
|
|
.. _tut-startup:
|
|
|
|
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 :envvar:`PYTHONSTARTUP` to the name of a
|
|
file containing your start-up commands. This is similar to the :file:`.profile`
|
|
feature of the Unix shells.
|
|
|
|
.. XXX This should probably be dumped in an appendix, since most people
|
|
don't use Python interactively in non-trivial ways.
|
|
|
|
This file is only read in interactive sessions, not when Python reads commands
|
|
from a script, and not when :file:`/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'): exec(open('.pythonrc.py').read())``.
|
|
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):
|
|
exec(open(filename).read())
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] A problem with the GNU Readline package may prevent this.
|
|
|