| .. _tut-using: |
| |
| **************************** |
| Using the Python Interpreter |
| **************************** |
| |
| |
| .. _tut-invoking: |
| |
| Invoking the Interpreter |
| ======================== |
| |
| The Python interpreter is usually installed as :file:`/usr/local/bin/python3.5` |
| 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 |
| |
| python3.5 |
| |
| 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:\\Python35`, 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 the command prompt in a DOS box:: |
| |
| set path=%path%;C:\python35 |
| |
| 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 |
| 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. |
| |
| |
| .. _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:: |
| |
| $ python3.5 |
| Python 3.5 (default, Sep 16 2015, 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. |
| |
| It is also possible to specify a different encoding for source files. In order |
| to do this, put one more special comment line right after the ``#!`` line to |
| define the source file encoding:: |
| |
| # -*- coding: encoding -*- |
| |
| With that declaration, everything in the source file will be treated as having |
| the encoding *encoding* instead of UTF-8. The list of possible encodings can be |
| found in the Python Library Reference, in the section on :mod:`codecs`. |
| |
| For example, if your editor of choice does not support UTF-8 encoded files and |
| insists on using some other encoding, say Windows-1252, you can write:: |
| |
| # -*- coding: cp-1252 -*- |
| |
| and still use all characters in the Windows-1252 character set in the source |
| files. The special encoding comment must be in the *first or second* line |
| within the file. |
| |
| |
| .. 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. |