164 lines
6.4 KiB
ReStructuredText
164 lines
6.4 KiB
ReStructuredText
.. _tut-using:
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****************************
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Using the Python Interpreter
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****************************
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.. _tut-invoking:
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Invoking the Interpreter
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========================
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The Python interpreter is usually installed as :file:`/usr/local/bin/python3.9`
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on those machines where it is available; putting :file:`/usr/local/bin` in your
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Unix shell's search path makes it possible to start it by typing the command:
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.. code-block:: text
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python3.9
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to the shell. [#]_ Since the choice of the directory where the interpreter lives
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is an installation option, other places are possible; check with your local
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Python guru or system administrator. (E.g., :file:`/usr/local/python` is a
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popular alternative location.)
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On Windows machines where you have installed from the :ref:`Microsoft Store
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<windows-store>`, the :file:`python3.9` command will be available. If you have
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the :ref:`py.exe launcher <launcher>` installed, you can use the :file:`py`
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command. See :ref:`setting-envvars` for other ways to launch Python.
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Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on
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Windows) at the primary prompt causes the interpreter to exit with a zero exit
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status. If that doesn't work, you can exit the interpreter by typing the
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following command: ``quit()``.
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The interpreter's line-editing features include interactive editing, history
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substitution and code completion on systems that support the `GNU Readline
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<https://tiswww.case.edu/php/chet/readline/rltop.html>`_ library.
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Perhaps the quickest check to see whether command line editing is supported is
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typing :kbd:`Control-P` to the first Python prompt you get. If it beeps, you
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have command line editing; see Appendix :ref:`tut-interacting` for an
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introduction to the keys. If nothing appears to happen, or if ``^P`` is
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echoed, command line editing isn't available; you'll only be able to use
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backspace to remove characters from the current line.
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The interpreter operates somewhat like the Unix shell: when called with standard
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input connected to a tty device, it reads and executes commands interactively;
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when called with a file name argument or with a file as standard input, it reads
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and executes a *script* from that file.
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A second way of starting the interpreter is ``python -c command [arg] ...``,
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which executes the statement(s) in *command*, analogous to the shell's
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:option:`-c` option. Since Python statements often contain spaces or other
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characters that are special to the shell, it is usually advised to quote
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*command* in its entirety with single quotes.
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Some Python modules are also useful as scripts. These can be invoked using
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``python -m module [arg] ...``, which executes the source file for *module* as
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if you had spelled out its full name on the command line.
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When a script file is used, it is sometimes useful to be able to run the script
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and enter interactive mode afterwards. This can be done by passing :option:`-i`
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before the script.
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All command line options are described in :ref:`using-on-general`.
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.. _tut-argpassing:
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Argument Passing
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----------------
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When known to the interpreter, the script name and additional arguments
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thereafter are turned into a list of strings and assigned to the ``argv``
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variable in the ``sys`` module. You can access this list by executing ``import
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sys``. The length of the list is at least one; when no script and no arguments
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are given, ``sys.argv[0]`` is an empty string. When the script name is given as
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``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When
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:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When
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:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the
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located module. Options found after :option:`-c` *command* or :option:`-m`
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*module* are not consumed by the Python interpreter's option processing but
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left in ``sys.argv`` for the command or module to handle.
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.. _tut-interactive:
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Interactive Mode
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----------------
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When commands are read from a tty, the interpreter is said to be in *interactive
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mode*. In this mode it prompts for the next command with the *primary prompt*,
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usually three greater-than signs (``>>>``); for continuation lines it prompts
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with the *secondary prompt*, by default three dots (``...``). The interpreter
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prints a welcome message stating its version number and a copyright notice
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before printing the first prompt:
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.. code-block:: shell-session
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$ python3.9
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Python 3.9 (default, June 4 2019, 09:25:04)
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[GCC 4.8.2] on linux
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Type "help", "copyright", "credits" or "license" for more information.
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>>>
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.. XXX update for new releases
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Continuation lines are needed when entering a multi-line construct. As an
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example, take a look at this :keyword:`if` statement::
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>>> the_world_is_flat = True
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>>> if the_world_is_flat:
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... print("Be careful not to fall off!")
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...
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Be careful not to fall off!
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For more on interactive mode, see :ref:`tut-interac`.
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.. _tut-interp:
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The Interpreter and Its Environment
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===================================
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.. _tut-source-encoding:
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Source Code Encoding
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--------------------
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By default, Python source files are treated as encoded in UTF-8. In that
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encoding, characters of most languages in the world can be used simultaneously
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in string literals, identifiers and comments --- although the standard library
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only uses ASCII characters for identifiers, a convention that any portable code
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should follow. To display all these characters properly, your editor must
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recognize that the file is UTF-8, and it must use a font that supports all the
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characters in the file.
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To declare an encoding other than the default one, a special comment line
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should be added as the *first* line of the file. The syntax is as follows::
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# -*- coding: encoding -*-
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where *encoding* is one of the valid :mod:`codecs` supported by Python.
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For example, to declare that Windows-1252 encoding is to be used, the first
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line of your source code file should be::
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# -*- coding: cp1252 -*-
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One exception to the *first line* rule is when the source code starts with a
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:ref:`UNIX "shebang" line <tut-scripts>`. In this case, the encoding
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declaration should be added as the second line of the file. For example::
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#!/usr/bin/env python3
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# -*- coding: cp1252 -*-
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.. rubric:: Footnotes
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.. [#] On Unix, the Python 3.x interpreter is by default not installed with the
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executable named ``python``, so that it does not conflict with a
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simultaneously installed Python 2.x executable.
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