1 .. _tut-using: 2 3 **************************** 4 Using the Python Interpreter 5 **************************** 6 7 8 .. _tut-invoking: 9 10 Invoking the Interpreter 11 ======================== 12 13 The Python interpreter is usually installed as :file:`/usr/local/bin/python` on 14 those machines where it is available; putting :file:`/usr/local/bin` in your 15 Unix shell's search path makes it possible to start it by typing the command :: 16 17 python 18 19 to the shell. Since the choice of the directory where the interpreter lives is 20 an installation option, other places are possible; check with your local Python 21 guru or system administrator. (E.g., :file:`/usr/local/python` is a popular 22 alternative location.) 23 24 On Windows machines, the Python installation is usually placed in 25 :file:`C:\\Python27`, though you can change this when you're running the 26 installer. To add this directory to your path, you can type the following 27 command into the command prompt in a DOS box:: 28 29 set path=%path%;C:\python27 30 31 Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on 32 Windows) at the primary prompt causes the interpreter to exit with a zero exit 33 status. If that doesn't work, you can exit the interpreter by typing the 34 following command: ``quit()``. 35 36 The interpreter's line-editing features usually aren't very sophisticated. On 37 Unix, whoever installed the interpreter may have enabled support for the GNU 38 readline library, which adds more elaborate interactive editing and history 39 features. Perhaps the quickest check to see whether command line editing is 40 supported is typing :kbd:`Control-P` to the first Python prompt you get. If it beeps, 41 you have command line editing; see Appendix :ref:`tut-interacting` for an 42 introduction to the keys. If nothing appears to happen, or if ``^P`` is echoed, 43 command line editing isn't available; you'll only be able to use backspace to 44 remove characters from the current line. 45 46 The interpreter operates somewhat like the Unix shell: when called with standard 47 input connected to a tty device, it reads and executes commands interactively; 48 when called with a file name argument or with a file as standard input, it reads 49 and executes a *script* from that file. 50 51 A second way of starting the interpreter is ``python -c command [arg] ...``, 52 which executes the statement(s) in *command*, analogous to the shell's 53 :option:`-c` option. Since Python statements often contain spaces or other 54 characters that are special to the shell, it is usually advised to quote 55 *command* in its entirety with single quotes. 56 57 Some 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 59 if you had spelled out its full name on the command line. 60 61 When a script file is used, it is sometimes useful to be able to run the script 62 and enter interactive mode afterwards. This can be done by passing :option:`-i` 63 before the script. 64 65 All command-line options are described in :ref:`using-on-general`. 66 67 68 .. _tut-argpassing: 69 70 Argument Passing 71 ---------------- 72 73 When known to the interpreter, the script name and additional arguments 74 thereafter are turned into a list of strings and assigned to the ``argv`` 75 variable in the ``sys`` module. You can access this list by executing ``import 76 sys``. The length of the list is at least one; when no script and no arguments 77 are 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 81 located module. Options found after :option:`-c` *command* or :option:`-m` 82 *module* are not consumed by the Python interpreter's option processing but 83 left in ``sys.argv`` for the command or module to handle. 84 85 86 .. _tut-interactive: 87 88 Interactive Mode 89 ---------------- 90 91 When commands are read from a tty, the interpreter is said to be in *interactive 92 mode*. In this mode it prompts for the next command with the *primary prompt*, 93 usually three greater-than signs (``>>>``); for continuation lines it prompts 94 with the *secondary prompt*, by default three dots (``...``). The interpreter 95 prints a welcome message stating its version number and a copyright notice 96 before 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 >>> 104 105 Continuation lines are needed when entering a multi-line construct. As an 106 example, 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! 113 114 115 For more on interactive mode, see :ref:`tut-interac`. 116 117 118 .. _tut-interp: 119 120 The Interpreter and Its Environment 121 =================================== 122 123 124 .. _tut-source-encoding: 125 126 Source Code Encoding 127 -------------------- 128 129 It is possible to use encodings different than ASCII in Python source files. The 130 best way to do it is to put one more special comment line right after the ``#!`` 131 line to define the source file encoding:: 132 133 # -*- coding: encoding -*- 134 135 136 With that declaration, all characters in the source file will be treated as 137 having the encoding *encoding*, and it will be possible to directly write 138 Unicode string literals in the selected encoding. The list of possible 139 encodings can be found in the Python Library Reference, in the section on 140 :mod:`codecs`. 141 142 For example, to write Unicode literals including the Euro currency symbol, the 143 ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal value 144 164. This script, when saved in the ISO-8859-15 encoding, will print the value 145 8364 (the Unicode code point corresponding to the Euro symbol) and then exit:: 146 147 # -*- coding: iso-8859-15 -*- 148 149 currency = u"" 150 print ord(currency) 151 152 If your editor supports saving files as ``UTF-8`` with a UTF-8 *byte order mark* 153 (aka BOM), you can use that instead of an encoding declaration. IDLE supports 154 this capability if ``Options/General/Default Source Encoding/UTF-8`` is set. 155 Notice that this signature is not understood in older Python releases (2.2 and 156 earlier), and also not understood by the operating system for script files with 157 ``#!`` lines (only used on Unix systems). 158 159 By using UTF-8 (either through the signature or an encoding declaration), 160 characters of most languages in the world can be used simultaneously in string 161 literals and comments. Using non-ASCII characters in identifiers is not 162 supported. To display all these characters properly, your editor must recognize 163 that the file is UTF-8, and it must use a font that supports all the characters 164 in the file. 165 166
