xref: /aosp_15_r20/external/python/cpython2/Doc/library/tkinter.rst

:mod:`Tkinter` --- Python interface to Tcl/Tk
=============================================

.. module:: Tkinter
   :synopsis: Interface to Tcl/Tk for graphical user interfaces
.. moduleauthor:: Guido van Rossum <[email protected]>


The :mod:`Tkinter` module ("Tk interface") is the standard Python interface to
the Tk GUI toolkit.  Both Tk and :mod:`Tkinter` are available on most Unix
platforms, as well as on Windows systems.  (Tk itself is not part of Python; it
is maintained at ActiveState.)

Running ``python -m Tkinter`` from the command line should open a window
demonstrating a simple Tk interface, letting you know that :mod:`Tkinter` is
properly installed on your system, and also showing what version of Tcl/Tk is
installed, so you can read the Tcl/Tk documentation specific to that version.

.. note::

   :mod:`Tkinter` has been renamed to :mod:`tkinter` in Python 3.  The
   :term:`2to3` tool will automatically adapt imports when converting your
   sources to Python 3.

.. seealso::

   Tkinter documentation:

   `Python Tkinter Resources <https://wiki.python.org/moin/TkInter>`_
      The Python Tkinter Topic Guide provides a great deal of information on using Tk
      from Python and links to other sources of information on Tk.

   `TKDocs <http://www.tkdocs.com/>`_
      Extensive tutorial plus friendlier widget pages for some of the widgets.

   `Tkinter reference: a GUI for Python <https://infohost.nmt.edu/tcc/help/pubs/tkinter/web/index.html>`_
      On-line reference material.

   `Tkinter docs from effbot <http://effbot.org/tkinterbook/>`_
      Online reference for tkinter supported by effbot.org.

   `Programming Python <http://learning-python.com/about-pp4e.html>`_
      Book by Mark Lutz, has excellent coverage of Tkinter.

   `Modern Tkinter for Busy Python Developers <https://www.amazon.com/Modern-Tkinter-Python-Developers-ebook/dp/B0071QDNLO/>`_
      Book by Mark Rozerman about building attractive and modern graphical user interfaces with Python and Tkinter.

   `Python and Tkinter Programming <https://www.manning.com/books/python-and-tkinter-programming>`_
      Book by John Grayson (ISBN 1-884777-81-3).

   Tcl/Tk documentation:

   `Tk commands <https://www.tcl.tk/man/tcl8.6/TkCmd/contents.htm>`_
      Most commands are available as :mod:`Tkinter` or :mod:`Tkinter.ttk` classes.
      Change '8.6' to match the version of your Tcl/Tk installation.

   `Tcl/Tk recent man pages <https://www.tcl.tk/doc/>`_
      Recent Tcl/Tk manuals on www.tcl.tk.

   `ActiveState Tcl Home Page <http://tcl.activestate.com/>`_
      The Tk/Tcl development is largely taking place at ActiveState.

   `Tcl and the Tk Toolkit <https://www.amazon.com/exec/obidos/ASIN/020163337X>`_
      Book by John Ousterhout, the inventor of Tcl.

   `Practical Programming in Tcl and Tk <http://www.beedub.com/book/>`_
      Brent Welch's encyclopedic book.


Tkinter Modules
---------------

Most of the time, the :mod:`Tkinter` module is all you really need, but a number
of additional modules are available as well.  The Tk interface is located in a
binary module named :mod:`_tkinter`. This module contains the low-level
interface to Tk, and should never be used directly by application programmers.
It is usually a shared library (or DLL), but might in some cases be statically
linked with the Python interpreter.

In addition to the Tk interface module, :mod:`Tkinter` includes a number of
Python modules. The two most important modules are the :mod:`Tkinter` module
itself, and a module called :mod:`Tkconstants`. The former automatically imports
the latter, so to use Tkinter, all you need to do is to import one module::

   import Tkinter

Or, more often::

   from Tkinter import *


.. class:: Tk(screenName=None, baseName=None, className='Tk', useTk=1)

   The :class:`Tk` class is instantiated without arguments. This creates a toplevel
   widget of Tk which usually is the main window of an application. Each instance
   has its own associated Tcl interpreter.

   .. FIXME: The following keyword arguments are currently recognized:

   .. versionchanged:: 2.4
      The *useTk* parameter was added.


.. function:: Tcl(screenName=None, baseName=None, className='Tk', useTk=0)

   The :func:`Tcl` function is a factory function which creates an object much like
   that created by the :class:`Tk` class, except that it does not initialize the Tk
   subsystem.  This is most often useful when driving the Tcl interpreter in an
   environment where one doesn't want to create extraneous toplevel windows, or
   where one cannot (such as Unix/Linux systems without an X server).  An object
   created by the :func:`Tcl` object can have a Toplevel window created (and the Tk
   subsystem initialized) by calling its :meth:`loadtk` method.

   .. versionadded:: 2.4

Other modules that provide Tk support include:

:mod:`ScrolledText`
   Text widget with a vertical scroll bar built in.

:mod:`tkColorChooser`
   Dialog to let the user choose a color.

:mod:`tkCommonDialog`
   Base class for the dialogs defined in the other modules listed here.

:mod:`tkFileDialog`
   Common dialogs to allow the user to specify a file to open or save.

:mod:`tkFont`
   Utilities to help work with fonts.

:mod:`tkMessageBox`
   Access to standard Tk dialog boxes.

:mod:`tkSimpleDialog`
   Basic dialogs and convenience functions.

:mod:`Tkdnd`
   Drag-and-drop support for :mod:`Tkinter`. This is experimental and should become
   deprecated when it is replaced  with the Tk DND.

:mod:`turtle`
   Turtle graphics in a Tk window.

These have been renamed as well in Python 3; they were all made submodules of
the new ``tkinter`` package.


Tkinter Life Preserver
----------------------

.. sectionauthor:: Matt Conway


This section is not designed to be an exhaustive tutorial on either Tk or
Tkinter.  Rather, it is intended as a stop gap, providing some introductory
orientation on the system.

Credits:

* Tkinter was written by Steen Lumholt and Guido van Rossum.

* Tk was written by John Ousterhout while at Berkeley.

* This Life Preserver was written by Matt Conway at the University of Virginia.

* The html rendering, and some liberal editing, was produced from a FrameMaker
  version by Ken Manheimer.

* Fredrik Lundh elaborated and revised the class interface descriptions, to get
  them current with Tk 4.2.

* Mike Clarkson converted the documentation to LaTeX, and compiled the  User
  Interface chapter of the reference manual.


How To Use This Section
^^^^^^^^^^^^^^^^^^^^^^^

This section is designed in two parts: the first half (roughly) covers
background material, while the second half can be taken to the keyboard as a
handy reference.

When trying to answer questions of the form "how do I do blah", it is often best
to find out how to do "blah" in straight Tk, and then convert this back into the
corresponding :mod:`Tkinter` call. Python programmers can often guess at the
correct Python command by looking at the Tk documentation. This means that in
order to use Tkinter, you will have to know a little bit about Tk. This document
can't fulfill that role, so the best we can do is point you to the best
documentation that exists. Here are some hints:

* The authors strongly suggest getting a copy of the Tk man pages. Specifically,
  the man pages in the ``mann`` directory are most useful. The ``man3`` man pages
  describe the C interface to the Tk library and thus are not especially helpful
  for script writers.

* Addison-Wesley publishes a book called Tcl and the Tk Toolkit by John
  Ousterhout (ISBN 0-201-63337-X) which is a good introduction to Tcl and Tk for
  the novice.  The book is not exhaustive, and for many details it defers to the
  man pages.

* :file:`Tkinter.py` is a last resort for most, but can be a good place to go
  when nothing else makes sense.


A Simple Hello World Program
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

   from Tkinter import *

   class Application(Frame):
       def say_hi(self):
           print "hi there, everyone!"

       def createWidgets(self):
           self.QUIT = Button(self)
           self.QUIT["text"] = "QUIT"
           self.QUIT["fg"]   = "red"
           self.QUIT["command"] =  self.quit

           self.QUIT.pack({"side": "left"})

           self.hi_there = Button(self)
           self.hi_there["text"] = "Hello",
           self.hi_there["command"] = self.say_hi

           self.hi_there.pack({"side": "left"})

       def __init__(self, master=None):
           Frame.__init__(self, master)
           self.pack()
           self.createWidgets()

   root = Tk()
   app = Application(master=root)
   app.mainloop()
   root.destroy()


A (Very) Quick Look at Tcl/Tk
-----------------------------

The class hierarchy looks complicated, but in actual practice, application
programmers almost always refer to the classes at the very bottom of the
hierarchy.

Notes:

* These classes are provided for the purposes of organizing certain functions
  under one namespace. They aren't meant to be instantiated independently.

* The :class:`Tk` class is meant to be instantiated only once in an application.
  Application programmers need not instantiate one explicitly, the system creates
  one whenever any of the other classes are instantiated.

* The :class:`Widget` class is not meant to be instantiated, it is meant only
  for subclassing to make "real" widgets (in C++, this is called an 'abstract
  class').

To make use of this reference material, there will be times when you will need
to know how to read short passages of Tk and how to identify the various parts
of a Tk command.   (See section :ref:`tkinter-basic-mapping` for the
:mod:`Tkinter` equivalents of what's below.)

Tk scripts are Tcl programs.  Like all Tcl programs, Tk scripts are just lists
of tokens separated by spaces.  A Tk widget is just its *class*, the *options*
that help configure it, and the *actions* that make it do useful things.

To make a widget in Tk, the command is always of the form::

   classCommand newPathname options

*classCommand*
   denotes which kind of widget to make (a button, a label, a menu...)

*newPathname*
   is the new name for this widget.  All names in Tk must be unique.  To help
   enforce this, widgets in Tk are named with *pathnames*, just like files in a
   file system.  The top level widget, the *root*, is called ``.`` (period) and
   children are delimited by more periods.  For example,
   ``.myApp.controlPanel.okButton`` might be the name of a widget.

*options*
   configure the widget's appearance and in some cases, its behavior.  The options
   come in the form of a list of flags and values. Flags are preceded by a '-',
   like Unix shell command flags, and values are put in quotes if they are more
   than one word.

For example::

   button   .fred   -fg red -text "hi there"
      ^       ^     \_____________________/
      |       |                |
    class    new            options
   command  widget  (-opt val -opt val ...)

Once created, the pathname to the widget becomes a new command.  This new
*widget command* is the programmer's handle for getting the new widget to
perform some *action*.  In C, you'd express this as someAction(fred,
someOptions), in C++, you would express this as fred.someAction(someOptions),
and in Tk, you say::

   .fred someAction someOptions

Note that the object name, ``.fred``, starts with a dot.

As you'd expect, the legal values for *someAction* will depend on the widget's
class: ``.fred disable`` works if fred is a button (fred gets greyed out), but
does not work if fred is a label (disabling of labels is not supported in Tk).

The legal values of *someOptions* is action dependent.  Some actions, like
``disable``, require no arguments, others, like a text-entry box's ``delete``
command, would need arguments to specify what range of text to delete.


.. _tkinter-basic-mapping:

Mapping Basic Tk into Tkinter
-----------------------------

Class commands in Tk correspond to class constructors in Tkinter. ::

   button .fred                =====>  fred = Button()

The master of an object is implicit in the new name given to it at creation
time.  In Tkinter, masters are specified explicitly. ::

   button .panel.fred          =====>  fred = Button(panel)

The configuration options in Tk are given in lists of hyphened tags followed by
values.  In Tkinter, options are specified as keyword-arguments in the instance
constructor, and keyword-args for configure calls or as instance indices, in
dictionary style, for established instances.  See section
:ref:`tkinter-setting-options` on setting options. ::

   button .fred -fg red        =====>  fred = Button(panel, fg = "red")
   .fred configure -fg red     =====>  fred["fg"] = red
                               OR ==>  fred.config(fg = "red")

In Tk, to perform an action on a widget, use the widget name as a command, and
follow it with an action name, possibly with arguments (options).  In Tkinter,
you call methods on the class instance to invoke actions on the widget.  The
actions (methods) that a given widget can perform are listed in the Tkinter.py
module. ::

   .fred invoke                =====>  fred.invoke()

To give a widget to the packer (geometry manager), you call pack with optional
arguments.  In Tkinter, the Pack class holds all this functionality, and the
various forms of the pack command are implemented as methods.  All widgets in
:mod:`Tkinter` are subclassed from the Packer, and so inherit all the packing
methods. See the :mod:`Tix` module documentation for additional information on
the Form geometry manager. ::

   pack .fred -side left       =====>  fred.pack(side = "left")


How Tk and Tkinter are Related
------------------------------

From the top down:

Your App Here (Python)
   A Python application makes a :mod:`Tkinter` call.

Tkinter (Python Module)
   This call (say, for example, creating a button widget), is implemented in the
   *Tkinter* module, which is written in Python.  This Python function will parse
   the commands and the arguments and convert them into a form that makes them look
   as if they had come from a Tk script instead of a Python script.

tkinter (C)
   These commands and their arguments will be passed to a C function in the
   *tkinter* - note the lowercase - extension module.

Tk Widgets (C and Tcl)
   This C function is able to make calls into other C modules, including the C
   functions that make up the Tk library.  Tk is implemented in C and some Tcl.
   The Tcl part of the Tk widgets is used to bind certain default behaviors to
   widgets, and is executed once at the point where the Python :mod:`Tkinter`
   module is imported. (The user never sees this stage).

Tk (C)
   The Tk part of the Tk Widgets implement the final mapping to ...

Xlib (C)
   the Xlib library to draw graphics on the screen.


Handy Reference
---------------


.. _tkinter-setting-options:

Setting Options
^^^^^^^^^^^^^^^

Options control things like the color and border width of a widget. Options can
be set in three ways:

At object creation time, using keyword arguments
   ::

      fred = Button(self, fg = "red", bg = "blue")

After object creation, treating the option name like a dictionary index
   ::

      fred["fg"] = "red"
      fred["bg"] = "blue"

Use the config() method to update multiple attrs subsequent to object creation
   ::

      fred.config(fg = "red", bg = "blue")

For a complete explanation of a given option and its behavior, see the Tk man
pages for the widget in question.

Note that the man pages list "STANDARD OPTIONS" and "WIDGET SPECIFIC OPTIONS"
for each widget.  The former is a list of options that are common to many
widgets, the latter are the options that are idiosyncratic to that particular
widget.  The Standard Options are documented on the :manpage:`options(3)` man
page.

No distinction between standard and widget-specific options is made in this
document.  Some options don't apply to some kinds of widgets. Whether a given
widget responds to a particular option depends on the class of the widget;
buttons have a ``command`` option, labels do not.

The options supported by a given widget are listed in that widget's man page, or
can be queried at runtime by calling the :meth:`config` method without
arguments, or by calling the :meth:`keys` method on that widget.  The return
value of these calls is a dictionary whose key is the name of the option as a
string (for example, ``'relief'``) and whose values are 5-tuples.

Some options, like ``bg`` are synonyms for common options with long names
(``bg`` is shorthand for "background"). Passing the ``config()`` method the name
of a shorthand option will return a 2-tuple, not 5-tuple. The 2-tuple passed
back will contain the name of the synonym and the "real" option (such as
``('bg', 'background')``).

+-------+---------------------------------+--------------+
| Index | Meaning                         | Example      |
+=======+=================================+==============+
| 0     | option name                     | ``'relief'`` |
+-------+---------------------------------+--------------+
| 1     | option name for database lookup | ``'relief'`` |
+-------+---------------------------------+--------------+
| 2     | option class for database       | ``'Relief'`` |
|       | lookup                          |              |
+-------+---------------------------------+--------------+
| 3     | default value                   | ``'raised'`` |
+-------+---------------------------------+--------------+
| 4     | current value                   | ``'groove'`` |
+-------+---------------------------------+--------------+

Example::

   >>> print fred.config()
   {'relief': ('relief', 'relief', 'Relief', 'raised', 'groove')}

Of course, the dictionary printed will include all the options available and
their values.  This is meant only as an example.


The Packer
^^^^^^^^^^

.. index:: single: packing (widgets)

The packer is one of Tk's geometry-management mechanisms.    Geometry managers
are used to specify the relative positioning of the positioning of widgets
within their container - their mutual *master*.  In contrast to the more
cumbersome *placer* (which is used less commonly, and we do not cover here), the
packer takes qualitative relationship specification - *above*, *to the left of*,
*filling*, etc - and works everything out to determine the exact placement
coordinates for you.

The size of any *master* widget is determined by the size of the "slave widgets"
inside.  The packer is used to control where slave widgets appear inside the
master into which they are packed.  You can pack widgets into frames, and frames
into other frames, in order to achieve the kind of layout you desire.
Additionally, the arrangement is dynamically adjusted to accommodate incremental
changes to the configuration, once it is packed.

Note that widgets do not appear until they have had their geometry specified
with a geometry manager.  It's a common early mistake to leave out the geometry
specification, and then be surprised when the widget is created but nothing
appears.  A widget will appear only after it has had, for example, the packer's
:meth:`pack` method applied to it.

The pack() method can be called with keyword-option/value pairs that control
where the widget is to appear within its container, and how it is to behave when
the main application window is resized.  Here are some examples::

   fred.pack()                     # defaults to side = "top"
   fred.pack(side = "left")
   fred.pack(expand = 1)


Packer Options
^^^^^^^^^^^^^^

For more extensive information on the packer and the options that it can take,
see the man pages and page 183 of John Ousterhout's book.

anchor
   Anchor type.  Denotes where the packer is to place each slave in its parcel.

expand
   Boolean, ``0`` or ``1``.

fill
   Legal values: ``'x'``, ``'y'``, ``'both'``, ``'none'``.

ipadx and ipady
   A distance - designating internal padding on each side of the slave widget.

padx and pady
   A distance - designating external padding on each side of the slave widget.

side
   Legal values are: ``'left'``, ``'right'``, ``'top'``, ``'bottom'``.


Coupling Widget Variables
^^^^^^^^^^^^^^^^^^^^^^^^^

The current-value setting of some widgets (like text entry widgets) can be
connected directly to application variables by using special options.  These
options are ``variable``, ``textvariable``, ``onvalue``, ``offvalue``, and
``value``.  This connection works both ways: if the variable changes for any
reason, the widget it's connected to will be updated to reflect the new value.

Unfortunately, in the current implementation of :mod:`Tkinter` it is not
possible to hand over an arbitrary Python variable to a widget through a
``variable`` or ``textvariable`` option.  The only kinds of variables for which
this works are variables that are subclassed from a class called Variable,
defined in the :mod:`Tkinter` module.

There are many useful subclasses of Variable already defined:
:class:`StringVar`, :class:`IntVar`, :class:`DoubleVar`, and
:class:`BooleanVar`.  To read the current value of such a variable, call the
:meth:`get` method on it, and to change its value you call the :meth:`!set`
method.  If you follow this protocol, the widget will always track the value of
the variable, with no further intervention on your part.

For example::

   class App(Frame):
       def __init__(self, master=None):
           Frame.__init__(self, master)
           self.pack()

           self.entrythingy = Entry()
           self.entrythingy.pack()

           # here is the application variable
           self.contents = StringVar()
           # set it to some value
           self.contents.set("this is a variable")
           # tell the entry widget to watch this variable
           self.entrythingy["textvariable"] = self.contents

           # and here we get a callback when the user hits return.
           # we will have the program print out the value of the
           # application variable when the user hits return
           self.entrythingy.bind('<Key-Return>',
                                 self.print_contents)

       def print_contents(self, event):
           print "hi. contents of entry is now ---->", \
                 self.contents.get()


The Window Manager
^^^^^^^^^^^^^^^^^^

.. index:: single: window manager (widgets)

In Tk, there is a utility command, ``wm``, for interacting with the window
manager.  Options to the ``wm`` command allow you to control things like titles,
placement, icon bitmaps, and the like.  In :mod:`Tkinter`, these commands have
been implemented as methods on the :class:`Wm` class.  Toplevel widgets are
subclassed from the :class:`Wm` class, and so can call the :class:`Wm` methods
directly.

To get at the toplevel window that contains a given widget, you can often just
refer to the widget's master.  Of course if the widget has been packed inside of
a frame, the master won't represent a toplevel window.  To get at the toplevel
window that contains an arbitrary widget, you can call the :meth:`_root` method.
This method begins with an underscore to denote the fact that this function is
part of the implementation, and not an interface to Tk functionality.

Here are some examples of typical usage::

   from Tkinter import *
   class App(Frame):
       def __init__(self, master=None):
           Frame.__init__(self, master)
           self.pack()


   # create the application
   myapp = App()

   #
   # here are method calls to the window manager class
   #
   myapp.master.title("My Do-Nothing Application")
   myapp.master.maxsize(1000, 400)

   # start the program
   myapp.mainloop()


Tk Option Data Types
^^^^^^^^^^^^^^^^^^^^

.. index:: single: Tk Option Data Types

anchor
   Legal values are points of the compass: ``"n"``, ``"ne"``, ``"e"``, ``"se"``,
   ``"s"``, ``"sw"``, ``"w"``, ``"nw"``, and also ``"center"``.

bitmap
   There are eight built-in, named bitmaps: ``'error'``, ``'gray25'``,
   ``'gray50'``, ``'hourglass'``, ``'info'``, ``'questhead'``, ``'question'``,
   ``'warning'``.  To specify an X bitmap filename, give the full path to the file,
   preceded with an ``@``, as in ``"@/usr/contrib/bitmap/gumby.bit"``.

boolean
   You can pass integers 0 or 1 or the strings ``"yes"`` or ``"no"``.

callback
   This is any Python function that takes no arguments.  For example::

      def print_it():
              print "hi there"
      fred["command"] = print_it

color
   Colors can be given as the names of X colors in the rgb.txt file, or as strings
   representing RGB values in 4 bit: ``"#RGB"``, 8 bit: ``"#RRGGBB"``, 12 bit"
   ``"#RRRGGGBBB"``, or 16 bit ``"#RRRRGGGGBBBB"`` ranges, where R,G,B here
   represent any legal hex digit.  See page 160 of Ousterhout's book for details.

cursor
   The standard X cursor names from :file:`cursorfont.h` can be used, without the
   ``XC_`` prefix.  For example to get a hand cursor (:const:`XC_hand2`), use the
   string ``"hand2"``.  You can also specify a bitmap and mask file of your own.
   See page 179 of Ousterhout's book.

distance
   Screen distances can be specified in either pixels or absolute distances.
   Pixels are given as numbers and absolute distances as strings, with the trailing
   character denoting units: ``c`` for centimetres, ``i`` for inches, ``m`` for
   millimetres, ``p`` for printer's points.  For example, 3.5 inches is expressed
   as ``"3.5i"``.

font
   Tk uses a list font name format, such as ``{courier 10 bold}``. Font sizes with
   positive numbers are measured in points; sizes with negative numbers are
   measured in pixels.

geometry
   This is a string of the form ``widthxheight``, where width and height are
   measured in pixels for most widgets (in characters for widgets displaying text).
   For example: ``fred["geometry"] = "200x100"``.

justify
   Legal values are the strings: ``"left"``, ``"center"``, ``"right"``, and
   ``"fill"``.

region
   This is a string with four space-delimited elements, each of which is a legal
   distance (see above).  For example: ``"2 3 4 5"`` and ``"3i 2i 4.5i 2i"`` and
   ``"3c 2c 4c 10.43c"``  are all legal regions.

relief
   Determines what the border style of a widget will be.  Legal values are:
   ``"raised"``, ``"sunken"``, ``"flat"``, ``"groove"``, and ``"ridge"``.

scrollcommand
   This is almost always the :meth:`!set` method of some scrollbar widget, but can
   be any widget method that takes a single argument.   Refer to the file
   :file:`Demo/tkinter/matt/canvas-with-scrollbars.py` in the Python source
   distribution for an example.

wrap:
   Must be one of: ``"none"``, ``"char"``, or ``"word"``.


Bindings and Events
^^^^^^^^^^^^^^^^^^^

.. index::
   single: bind (widgets)
   single: events (widgets)

The bind method from the widget command allows you to watch for certain events
and to have a callback function trigger when that event type occurs.  The form
of the bind method is::

   def bind(self, sequence, func, add=''):

where:

sequence
   is a string that denotes the target kind of event.  (See the bind man page and
   page 201 of John Ousterhout's book for details).

func
   is a Python function, taking one argument, to be invoked when the event occurs.
   An Event instance will be passed as the argument. (Functions deployed this way
   are commonly known as *callbacks*.)

add
   is optional, either ``''`` or ``'+'``.  Passing an empty string denotes that
   this binding is to replace any other bindings that this event is associated
   with.  Passing a ``'+'`` means that this function is to be added to the list
   of functions bound to this event type.

For example::

   def turnRed(self, event):
       event.widget["activeforeground"] = "red"

   self.button.bind("<Enter>", self.turnRed)

Notice how the widget field of the event is being accessed in the
:meth:`turnRed` callback.  This field contains the widget that caught the X
event.  The following table lists the other event fields you can access, and how
they are denoted in Tk, which can be useful when referring to the Tk man pages.
::

   Tk      Tkinter Event Field             Tk      Tkinter Event Field
   --      -------------------             --      -------------------
   %f      focus                           %A      char
   %h      height                          %E      send_event
   %k      keycode                         %K      keysym
   %s      state                           %N      keysym_num
   %t      time                            %T      type
   %w      width                           %W      widget
   %x      x                               %X      x_root
   %y      y                               %Y      y_root


The index Parameter
^^^^^^^^^^^^^^^^^^^

A number of widgets require"index" parameters to be passed.  These are used to
point at a specific place in a Text widget, or to particular characters in an
Entry widget, or to particular menu items in a Menu widget.

Entry widget indexes (index, view index, etc.)
   Entry widgets have options that refer to character positions in the text being
   displayed.  You can use these :mod:`Tkinter` functions to access these special
   points in text widgets:

   AtEnd()
      refers to the last position in the text

   AtInsert()
      refers to the point where the text cursor is

   AtSelFirst()
      indicates the beginning point of the selected text

   AtSelLast()
      denotes the last point of the selected text and finally

   At(x[, y])
      refers to the character at pixel location *x*, *y* (with *y* not used in the
      case of a text entry widget, which contains a single line of text).

Text widget indexes
   The index notation for Text widgets is very rich and is best described in the Tk
   man pages.

Menu indexes (menu.invoke(), menu.entryconfig(), etc.)
   Some options and methods for menus manipulate specific menu entries. Anytime a
   menu index is needed for an option or a parameter, you may pass in:

   * an integer which refers to the numeric position of the entry in the widget,
     counted from the top, starting with 0;

   * the string ``'active'``, which refers to the menu position that is currently
     under the cursor;

   * the string ``"last"`` which refers to the last menu item;

   * An integer preceded by ``@``, as in ``@6``, where the integer is interpreted
     as a y pixel coordinate in the menu's coordinate system;

   * the string ``"none"``, which indicates no menu entry at all, most often used
     with menu.activate() to deactivate all entries, and finally,

   * a text string that is pattern matched against the label of the menu entry, as
     scanned from the top of the menu to the bottom.  Note that this index type is
     considered after all the others, which means that matches for menu items
     labelled ``last``, ``active``, or ``none`` may be interpreted as the above
     literals, instead.


Images
^^^^^^

Images of different formats can be created through the corresponding subclass
of :class:`Tkinter.Image`:

* :class:`BitmapImage` for images in XBM format.

* :class:`PhotoImage` for images in PGM, PPM, GIF and PNG formats. The latter
  is supported starting with Tk 8.6.

Either type of image is created through either the ``file`` or the ``data``
option (other options are available as well).

The image object can then be used wherever an ``image`` option is supported by
some widget (e.g. labels, buttons, menus). In these cases, Tk will not keep a
reference to the image. When the last Python reference to the image object is
deleted, the image data is deleted as well, and Tk will display an empty box
wherever the image was used.

.. seealso::

    The `Pillow <http://python-pillow.org/>`_ package adds support for
    formats such as BMP, JPEG, TIFF, and WebP, among others.

.. _tkinter-file-handlers:

File Handlers
-------------

Tk allows you to register and unregister a callback function which will be
called from the Tk mainloop when I/O is possible on a file descriptor.
Only one handler may be registered per file descriptor. Example code::

   import Tkinter
   widget = Tkinter.Tk()
   mask = Tkinter.READABLE | Tkinter.WRITABLE
   widget.tk.createfilehandler(file, mask, callback)
   ...
   widget.tk.deletefilehandler(file)

This feature is not available on Windows.

Since you don't know how many bytes are available for reading, you may not
want to use the :class:`~io.BufferedIOBase` or :class:`~io.TextIOBase`
:meth:`~io.BufferedIOBase.read` or :meth:`~io.IOBase.readline` methods,
since these will insist on reading a predefined number of bytes.
For sockets, the :meth:`~socket.socket.recv` or
:meth:`~socket.socket.recvfrom` methods will work fine; for other files,
use raw reads or ``os.read(file.fileno(), maxbytecount)``.


.. method:: Widget.tk.createfilehandler(file, mask, func)

   Registers the file handler callback function *func*. The *file* argument
   may either be an object with a :meth:`~io.IOBase.fileno` method (such as
   a file or socket object), or an integer file descriptor. The *mask*
   argument is an ORed combination of any of the three constants below.
   The callback is called as follows::

      callback(file, mask)


.. method:: Widget.tk.deletefilehandler(file)

   Unregisters a file handler.


.. data:: READABLE
          WRITABLE
          EXCEPTION

   Constants used in the *mask* arguments.