.. _tut-using: **************************** Using the Python Interpreter **************************** .. _tut-invoking: Invoking the Interpreter ======================== The Python interpreter is usually installed as :file:`/usr/local/bin/python3.9` on those machines where it is available; putting :file:`/usr/local/bin` in your Unix shell's search path makes it possible to start it by typing the command: .. code-block:: text python3.9 to the shell. [#]_ Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., :file:`/usr/local/python` is a popular alternative location.) On Windows machines, the Python installation is usually placed in :file:`C:\\Python38`, though you can change this when you're running the installer. To add this directory to your path, you can type the following command into :ref:`a command prompt window `:: set path=%path%;C:\python38 Typing an end-of-file character (:kbd:`Control-D` on Unix, :kbd:`Control-Z` on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn't work, you can exit the interpreter by typing the following command: ``quit()``. The interpreter's line-editing features include interactive editing, history substitution and code completion on systems that support readline. Perhaps the quickest check to see whether command line editing is supported is typing :kbd:`Control-P` to the first Python prompt you get. If it beeps, you have command line editing; see Appendix :ref:`tut-interacting` for an introduction to the keys. If nothing appears to happen, or if ``^P`` is echoed, command line editing isn't available; you'll only be able to use backspace to remove characters from the current line. The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a *script* from that file. A second way of starting the interpreter is ``python -c command [arg] ...``, which executes the statement(s) in *command*, analogous to the shell's :option:`-c` option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote *command* in its entirety with single quotes. Some Python modules are also useful as scripts. These can be invoked using ``python -m module [arg] ...``, which executes the source file for *module* as if you had spelled out its full name on the command line. When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing :option:`-i` before the script. All command line options are described in :ref:`using-on-general`. .. _tut-argpassing: Argument Passing ---------------- When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the ``argv`` variable in the ``sys`` module. You can access this list by executing ``import sys``. The length of the list is at least one; when no script and no arguments are given, ``sys.argv[0]`` is an empty string. When the script name is given as ``'-'`` (meaning standard input), ``sys.argv[0]`` is set to ``'-'``. When :option:`-c` *command* is used, ``sys.argv[0]`` is set to ``'-c'``. When :option:`-m` *module* is used, ``sys.argv[0]`` is set to the full name of the located module. Options found after :option:`-c` *command* or :option:`-m` *module* are not consumed by the Python interpreter's option processing but left in ``sys.argv`` for the command or module to handle. .. _tut-interactive: Interactive Mode ---------------- When commands are read from a tty, the interpreter is said to be in *interactive mode*. In this mode it prompts for the next command with the *primary prompt*, usually three greater-than signs (``>>>``); for continuation lines it prompts with the *secondary prompt*, by default three dots (``...``). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt: .. code-block:: shell-session $ python3.9 Python 3.9 (default, June 4 2019, 09:25:04) [GCC 4.8.2] on linux Type "help", "copyright", "credits" or "license" for more information. >>> .. XXX update for new releases Continuation lines are needed when entering a multi-line construct. As an example, take a look at this :keyword:`if` statement:: >>> the_world_is_flat = True >>> if the_world_is_flat: ... print("Be careful not to fall off!") ... Be careful not to fall off! For more on interactive mode, see :ref:`tut-interac`. .. _tut-interp: The Interpreter and Its Environment =================================== .. _tut-source-encoding: Source Code Encoding -------------------- By default, Python source files are treated as encoded in UTF-8. In that encoding, characters of most languages in the world can be used simultaneously in string literals, identifiers and comments --- although the standard library only uses ASCII characters for identifiers, a convention that any portable code should follow. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file. To declare an encoding other than the default one, a special comment line should be added as the *first* line of the file. The syntax is as follows:: # -*- coding: encoding -*- where *encoding* is one of the valid :mod:`codecs` supported by Python. For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:: # -*- coding: cp1252 -*- One exception to the *first line* rule is when the source code starts with a :ref:`UNIX "shebang" line `. In this case, the encoding declaration should be added as the second line of the file. For example:: #!/usr/bin/env python3 # -*- coding: cp1252 -*- .. rubric:: Footnotes .. [#] On Unix, the Python 3.x interpreter is by default not installed with the executable named ``python``, so that it does not conflict with a simultaneously installed Python 2.x executable.