1.. _tut-using: 2 3**************************** 4Using the Python Interpreter 5**************************** 6 7 8.. _tut-invoking: 9 10Invoking the Interpreter 11======================== 12 13The Python interpreter is usually installed as :file:`/usr/local/bin/python3.8` 14on those machines where it is available; putting :file:`/usr/local/bin` in your 15Unix shell's search path makes it possible to start it by typing the command: 16 17.. code-block:: text 18 19 python3.8 20 21to the shell. [#]_ Since the choice of the directory where the interpreter lives 22is an installation option, other places are possible; check with your local 23Python guru or system administrator. (E.g., :file:`/usr/local/python` is a 24popular alternative location.) 25 26On Windows machines where you have installed Python from the :ref:`Microsoft Store 27<windows-store>`, the :file:`python3.8` command will be available. If you have 28the :ref:`py.exe launcher <launcher>` installed, you can use the :file:`py` 29command. See :ref:`setting-envvars` for other ways to launch Python. 30 31Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on 32Windows) at the primary prompt causes the interpreter to exit with a zero exit 33status. If that doesn't work, you can exit the interpreter by typing the 34following command: ``quit()``. 35 36The interpreter's line-editing features include interactive editing, history 37substitution and code completion on systems that support the `GNU Readline 38<https://tiswww.case.edu/php/chet/readline/rltop.html>`_ library. 39Perhaps the quickest check to see whether command line editing is supported is 40typing :kbd:`Control-P` to the first Python prompt you get. If it beeps, you 41have command line editing; see Appendix :ref:`tut-interacting` for an 42introduction to the keys. If nothing appears to happen, or if ``^P`` is 43echoed, command line editing isn't available; you'll only be able to use 44backspace to remove characters from the current line. 45 46The interpreter operates somewhat like the Unix shell: when called with standard 47input connected to a tty device, it reads and executes commands interactively; 48when called with a file name argument or with a file as standard input, it reads 49and executes a *script* from that file. 50 51A second way of starting the interpreter is ``python -c command [arg] ...``, 52which executes the statement(s) in *command*, analogous to the shell's 53:option:`-c` option. Since Python statements often contain spaces or other 54characters that are special to the shell, it is usually advised to quote 55*command* in its entirety with single quotes. 56 57Some Python modules are also useful as scripts. These can be invoked using 58``python -m module [arg] ...``, which executes the source file for *module* as 59if you had spelled out its full name on the command line. 60 61When a script file is used, it is sometimes useful to be able to run the script 62and enter interactive mode afterwards. This can be done by passing :option:`-i` 63before the script. 64 65All command line options are described in :ref:`using-on-general`. 66 67 68.. _tut-argpassing: 69 70Argument Passing 71---------------- 72 73When known to the interpreter, the script name and additional arguments 74thereafter are turned into a list of strings and assigned to the ``argv`` 75variable in the ``sys`` module. You can access this list by executing ``import 76sys``. The length of the list is at least one; when no script and no arguments 77are given, ``sys.argv[0]`` is an empty string. When the script name is given as 78``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When 79:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When 80:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the 81located module. Options found after :option:`-c` *command* or :option:`-m` 82*module* are not consumed by the Python interpreter's option processing but 83left in ``sys.argv`` for the command or module to handle. 84 85 86.. _tut-interactive: 87 88Interactive Mode 89---------------- 90 91When commands are read from a tty, the interpreter is said to be in *interactive 92mode*. In this mode it prompts for the next command with the *primary prompt*, 93usually three greater-than signs (``>>>``); for continuation lines it prompts 94with the *secondary prompt*, by default three dots (``...``). The interpreter 95prints a welcome message stating its version number and a copyright notice 96before printing the first prompt: 97 98.. code-block:: shell-session 99 100 $ python3.8 101 Python 3.8 (default, Sep 16 2015, 09:25:04) 102 [GCC 4.8.2] on linux 103 Type "help", "copyright", "credits" or "license" for more information. 104 >>> 105 106.. XXX update for new releases 107 108Continuation lines are needed when entering a multi-line construct. As an 109example, take a look at this :keyword:`if` statement:: 110 111 >>> the_world_is_flat = True 112 >>> if the_world_is_flat: 113 ... print("Be careful not to fall off!") 114 ... 115 Be careful not to fall off! 116 117 118For more on interactive mode, see :ref:`tut-interac`. 119 120 121.. _tut-interp: 122 123The Interpreter and Its Environment 124=================================== 125 126 127.. _tut-source-encoding: 128 129Source Code Encoding 130-------------------- 131 132By default, Python source files are treated as encoded in UTF-8. In that 133encoding, characters of most languages in the world can be used simultaneously 134in string literals, identifiers and comments --- although the standard library 135only uses ASCII characters for identifiers, a convention that any portable code 136should follow. To display all these characters properly, your editor must 137recognize that the file is UTF-8, and it must use a font that supports all the 138characters in the file. 139 140To declare an encoding other than the default one, a special comment line 141should be added as the *first* line of the file. The syntax is as follows:: 142 143 # -*- coding: encoding -*- 144 145where *encoding* is one of the valid :mod:`codecs` supported by Python. 146 147For example, to declare that Windows-1252 encoding is to be used, the first 148line of your source code file should be:: 149 150 # -*- coding: cp1252 -*- 151 152One exception to the *first line* rule is when the source code starts with a 153:ref:`UNIX "shebang" line <tut-scripts>`. In this case, the encoding 154declaration should be added as the second line of the file. For example:: 155 156 #!/usr/bin/env python3 157 # -*- coding: cp1252 -*- 158 159.. rubric:: Footnotes 160 161.. [#] On Unix, the Python 3.x interpreter is by default not installed with the 162 executable named ``python``, so that it does not conflict with a 163 simultaneously installed Python 2.x executable. 164