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/python` on 14those 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 python 18 19to the shell. Since the choice of the directory where the interpreter lives is 20an installation option, other places are possible; check with your local Python 21guru or system administrator. (E.g., :file:`/usr/local/python` is a popular 22alternative location.) 23 24On Windows machines, the Python installation is usually placed in 25:file:`C:\\Python27`, though you can change this when you're running the 26installer. To add this directory to your path, you can type the following 27command into the command prompt in a DOS box:: 28 29 set path=%path%;C:\python27 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 usually aren't very sophisticated. On 37Unix, whoever installed the interpreter may have enabled support for the GNU 38readline library, which adds more elaborate interactive editing and history 39features. Perhaps the quickest check to see whether command line editing is 40supported is typing :kbd:`Control-P` to the first Python prompt you get. If it beeps, 41you have command line editing; see Appendix :ref:`tut-interacting` for an 42introduction to the keys. If nothing appears to happen, or if ``^P`` is echoed, 43command line editing isn't available; you'll only be able to use backspace to 44remove 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 python 101 Python 2.7 (#1, Feb 28 2010, 00:02:06) 102 Type "help", "copyright", "credits" or "license" for more information. 103 >>> 104 105Continuation lines are needed when entering a multi-line construct. As an 106example, take a look at this :keyword:`if` statement:: 107 108 >>> the_world_is_flat = 1 109 >>> if the_world_is_flat: 110 ... print "Be careful not to fall off!" 111 ... 112 Be careful not to fall off! 113 114 115For more on interactive mode, see :ref:`tut-interac`. 116 117 118.. _tut-interp: 119 120The Interpreter and Its Environment 121=================================== 122 123 124.. _tut-source-encoding: 125 126Source Code Encoding 127-------------------- 128 129By default, Python source files are treated as encoded in ASCII. 130To declare an encoding other than the default one, a special comment line 131should be added as the *first* line of the file. The syntax is as follows:: 132 133 # -*- coding: encoding -*- 134 135where *encoding* is one of the valid :mod:`codecs` supported by Python. 136 137For example, to declare that Windows-1252 encoding is to be used, the first 138line of your source code file should be:: 139 140 # -*- coding: cp1252 -*- 141 142One exception to the *first line* rule is when the source code starts with a 143:ref:`UNIX "shebang" line <tut-scripts>`. In this case, the encoding 144declaration should be added as the second line of the file. For example:: 145 146 #!/usr/bin/env python 147 # -*- coding: cp1252 -*- 148 149