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/python3.7` 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 python3.7 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, the Python installation is usually placed in 27:file:`C:\\Python37`, though you can change this when you're running the 28installer. To add this directory to your path, you can type the following 29command into the command prompt in a DOS box:: 30 31 set path=%path%;C:\python37 32 33Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on 34Windows) at the primary prompt causes the interpreter to exit with a zero exit 35status. If that doesn't work, you can exit the interpreter by typing the 36following command: ``quit()``. 37 38The interpreter's line-editing features include interactive editing, history 39substitution and code completion on systems that support readline. Perhaps the 40quickest check to see whether command line editing is supported is typing 41:kbd:`Control-P` to the first Python prompt you get. If it beeps, you have command 42line editing; see Appendix :ref:`tut-interacting` for an introduction to the 43keys. If nothing appears to happen, or if ``^P`` is echoed, command line 44editing isn't available; you'll only be able to use backspace to remove 45characters from the current line. 46 47The interpreter operates somewhat like the Unix shell: when called with standard 48input connected to a tty device, it reads and executes commands interactively; 49when called with a file name argument or with a file as standard input, it reads 50and executes a *script* from that file. 51 52A second way of starting the interpreter is ``python -c command [arg] ...``, 53which executes the statement(s) in *command*, analogous to the shell's 54:option:`-c` option. Since Python statements often contain spaces or other 55characters that are special to the shell, it is usually advised to quote 56*command* in its entirety with single quotes. 57 58Some Python modules are also useful as scripts. These can be invoked using 59``python -m module [arg] ...``, which executes the source file for *module* as 60if you had spelled out its full name on the command line. 61 62When a script file is used, it is sometimes useful to be able to run the script 63and enter interactive mode afterwards. This can be done by passing :option:`-i` 64before the script. 65 66All command line options are described in :ref:`using-on-general`. 67 68 69.. _tut-argpassing: 70 71Argument Passing 72---------------- 73 74When known to the interpreter, the script name and additional arguments 75thereafter are turned into a list of strings and assigned to the ``argv`` 76variable in the ``sys`` module. You can access this list by executing ``import 77sys``. The length of the list is at least one; when no script and no arguments 78are given, ``sys.argv[0]`` is an empty string. When the script name is given as 79``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When 80:option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When 81:option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the 82located module. Options found after :option:`-c` *command* or :option:`-m` 83*module* are not consumed by the Python interpreter's option processing but 84left in ``sys.argv`` for the command or module to handle. 85 86 87.. _tut-interactive: 88 89Interactive Mode 90---------------- 91 92When commands are read from a tty, the interpreter is said to be in *interactive 93mode*. In this mode it prompts for the next command with the *primary prompt*, 94usually three greater-than signs (``>>>``); for continuation lines it prompts 95with the *secondary prompt*, by default three dots (``...``). The interpreter 96prints a welcome message stating its version number and a copyright notice 97before printing the first prompt: 98 99.. code-block:: shell-session 100 101 $ python3.7 102 Python 3.7 (default, Sep 16 2015, 09:25:04) 103 [GCC 4.8.2] on linux 104 Type "help", "copyright", "credits" or "license" for more information. 105 >>> 106 107.. XXX update for new releases 108 109Continuation lines are needed when entering a multi-line construct. As an 110example, take a look at this :keyword:`if` statement:: 111 112 >>> the_world_is_flat = True 113 >>> if the_world_is_flat: 114 ... print("Be careful not to fall off!") 115 ... 116 Be careful not to fall off! 117 118 119For more on interactive mode, see :ref:`tut-interac`. 120 121 122.. _tut-interp: 123 124The Interpreter and Its Environment 125=================================== 126 127 128.. _tut-source-encoding: 129 130Source Code Encoding 131-------------------- 132 133By default, Python source files are treated as encoded in UTF-8. In that 134encoding, characters of most languages in the world can be used simultaneously 135in string literals, identifiers and comments --- although the standard library 136only uses ASCII characters for identifiers, a convention that any portable code 137should follow. To display all these characters properly, your editor must 138recognize that the file is UTF-8, and it must use a font that supports all the 139characters in the file. 140 141To declare an encoding other than the default one, a special comment line 142should be added as the *first* line of the file. The syntax is as follows:: 143 144 # -*- coding: encoding -*- 145 146where *encoding* is one of the valid :mod:`codecs` supported by Python. 147 148For example, to declare that Windows-1252 encoding is to be used, the first 149line of your source code file should be:: 150 151 # -*- coding: cp1252 -*- 152 153One exception to the *first line* rule is when the source code starts with a 154:ref:`UNIX "shebang" line <tut-scripts>`. In this case, the encoding 155declaration should be added as the second line of the file. For example:: 156 157 #!/usr/bin/env python3 158 # -*- coding: cp1252 -*- 159 160.. rubric:: Footnotes 161 162.. [#] On Unix, the Python 3.x interpreter is by default not installed with the 163 executable named ``python``, so that it does not conflict with a 164 simultaneously installed Python 2.x executable. 165