"tkinter" --- Python interface to Tcl/Tk
****************************************

**Source code:** Lib/tkinter/__init__.py

======================================================================

The "tkinter" package ("Tk interface") is the standard Python
interface to the Tcl/Tk GUI toolkit.  Both Tk and "tkinter" are
available on most Unix platforms, including macOS, as well as on
Windows systems.

Running "python -m tkinter" from the command line should open a window
demonstrating a simple Tk interface, letting you know that "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.

Tkinter supports a range of Tcl/Tk versions, built either with or
without thread support. The official Python binary release bundles
Tcl/Tk 8.6 threaded. See the source code for the "_tkinter" module for
more information about supported versions.

Tkinter is not a thin wrapper, but adds a fair amount of its own logic
to make the experience more pythonic. This documentation will
concentrate on these additions and changes, and refer to the official
Tcl/Tk documentation for details that are unchanged.

Note:

  Tcl/Tk 8.5 (2007) introduced a modern set of themed user interface
  components along with a new API to use them. Both old and new APIs
  are still available. Most documentation you will find online still
  uses the old API and can be woefully outdated.

See also:

  * TkDocs
       Extensive tutorial on creating user interfaces with Tkinter.
       Explains key concepts, and illustrates recommended approaches
       using the modern API.

  * Tkinter 8.5 reference: a GUI for Python
       Reference documentation for Tkinter 8.5 detailing available
       classes, methods, and options.

  Tcl/Tk Resources:

  * Tk commands
       Comprehensive reference to each of the underlying Tcl/Tk
       commands used by Tkinter.

  * Tcl/Tk Home Page
       Additional documentation, and links to Tcl/Tk core development.

  Books:

  * Modern Tkinter for Busy Python Developers
       By Mark Roseman. (ISBN 978-1999149567)

  * Python and Tkinter Programming
       By Alan Moore. (ISBN 978-1788835886)

  * Programming Python
       By Mark Lutz; has excellent coverage of Tkinter. (ISBN
       978-0596158101)

  * Tcl and the Tk Toolkit (2nd edition)
       By John Ousterhout, inventor of Tcl/Tk, and Ken Jones; does not
       cover Tkinter. (ISBN 978-0321336330)


Architecture
============

Tcl/Tk is not a single library but rather consists of a few distinct
modules, each with separate functionality and its own official
documentation. Python's binary releases also ship an add-on module
together with it.

Tcl
   Tcl is a dynamic interpreted programming language, just like
   Python. Though it can be used on its own as a general-purpose
   programming language, it is most commonly embedded into C
   applications as a scripting engine or an interface to the Tk
   toolkit. The Tcl library has a C interface to create and manage one
   or more instances of a Tcl interpreter, run Tcl commands and
   scripts in those instances, and add custom commands implemented in
   either Tcl or C. Each interpreter has an event queue, and there are
   facilities to send events to it and process them. Unlike Python,
   Tcl's execution model is designed around cooperative multitasking,
   and Tkinter bridges this difference (see Threading model for
   details).

Tk
   Tk is a Tcl package implemented in C that adds custom commands to
   create and manipulate GUI widgets. Each "Tk" object embeds its own
   Tcl interpreter instance with Tk loaded into it. Tk's widgets are
   very customizable, though at the cost of a dated appearance. Tk
   uses Tcl's event queue to generate and process GUI events.

Ttk
   Themed Tk (Ttk) is a newer family of Tk widgets that provide a much
   better appearance on different platforms than many of the classic
   Tk widgets. Ttk is distributed as part of Tk, starting with Tk
   version 8.5. Python bindings are provided in a separate module,
   "tkinter.ttk".

Internally, Tk and Ttk use facilities of the underlying operating
system, i.e., Xlib on Unix/X11, Cocoa on macOS, GDI on Windows.

When your Python application uses a class in Tkinter, e.g., to create
a widget, the "tkinter" module first assembles a Tcl/Tk command
string. It passes that Tcl command string to an internal "_tkinter"
binary module, which then calls the Tcl interpreter to evaluate it.
The Tcl interpreter will then call into the Tk and/or Ttk packages,
which will in turn make calls to Xlib, Cocoa, or GDI.


Tkinter Modules
===============

Support for Tkinter is spread across several modules. Most
applications will need the main "tkinter" module, as well as the
"tkinter.ttk" module, which provides the modern themed widget set and
API:

   from tkinter import *
   from tkinter import ttk

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

   The "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.

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

   The "Tcl()" function is a factory function which creates an object
   much like that created by the "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 "Tcl()" object can have a Toplevel window created
   (and the Tk subsystem initialized) by calling its "loadtk()"
   method.

The modules that provide Tk support include:

"tkinter"
   Main Tkinter module.

"tkinter.colorchooser"
   Dialog to let the user choose a color.

"tkinter.commondialog"
   Base class for the dialogs defined in the other modules listed
   here.

"tkinter.filedialog"
   Common dialogs to allow the user to specify a file to open or save.

"tkinter.font"
   Utilities to help work with fonts.

"tkinter.messagebox"
   Access to standard Tk dialog boxes.

"tkinter.scrolledtext"
   Text widget with a vertical scroll bar built in.

"tkinter.simpledialog"
   Basic dialogs and convenience functions.

"tkinter.ttk"
   Themed widget set introduced in Tk 8.5, providing modern
   alternatives for many of the classic widgets in the main "tkinter"
   module.

Additional modules:

"_tkinter"
   A binary module that contains the low-level interface to Tcl/Tk. It
   is automatically imported by the main "tkinter" module, 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.

"idlelib"
   Python's Integrated Development and Learning Environment (IDLE).
   Based on "tkinter".

"tkinter.constants"
   Symbolic constants that can be used in place of strings when
   passing various parameters to Tkinter calls. Automatically imported
   by the main "tkinter" module.

"tkinter.dnd"
   (experimental) Drag-and-drop support for "tkinter". This will
   become deprecated when it is replaced with the Tk DND.

"tkinter.tix"
   (deprecated) An older third-party Tcl/Tk package that adds several
   new widgets. Better alternatives for most can be found in
   "tkinter.ttk".

"turtle"
   Turtle graphics in a Tk window.


Tkinter Life Preserver
======================

This section is not designed to be an exhaustive tutorial on either Tk
or Tkinter.  For that, refer to one of the external resources noted
earlier. Instead, this section provides a very quick orientation to
what a Tkinter application looks like, identifies foundational Tk
concepts, and explains how the Tkinter wrapper is structured.

The remainder of this section will help you to identify the classes,
methods, and options you'll need in your Tkinter application, and
where to find more detailed documentation on them, including in the
official Tcl/Tk reference manual.


A Hello World Program
---------------------

We'll start by walking through a "Hello World" application in Tkinter.
This isn't the smallest one we could write, but has enough to
illustrate some key concepts you'll need to know.

   from tkinter import *
   from tkinter import ttk
   root = Tk()
   frm = ttk.Frame(root, padding=10)
   frm.grid()
   ttk.Label(frm, text="Hello World!").grid(column=0, row=0)
   ttk.Button(frm, text="Quit", command=root.destroy).grid(column=1, row=0)
   root.mainloop()

After the imports, the next line creates an instance of the "Tk"
class, which initializes Tk and creates its associated Tcl
interpreter. It also creates a toplevel window, known as the root
window, which serves as the main window of the application.

The following line creates a frame widget, which in this case will
contain a label and a button we'll create next. The frame is fit
inside the root window.

The next line creates a label widget holding a static text string. The
"grid()" method is used to specify the relative layout (position) of
the label within its containing frame widget, similar to how tables in
HTML work.

A button widget is then created, and placed to the right of the label.
When pressed, it will call the "destroy()" method of the root window.

Finally, the "mainloop()" method puts everything on the display, and
responds to user input until the program terminates.


Important Tk Concepts
---------------------

Even this simple program illustrates the following key Tk concepts:

widgets
   A Tkinter user interface is made up of individual *widgets*. Each
   widget is represented as a Python object, instantiated from classes
   like "ttk.Frame", "ttk.Label", and "ttk.Button".

widget hierarchy
   Widgets are arranged in a *hierarchy*. The label and button were
   contained within a frame, which in turn was contained within the
   root window. When creating each *child* widget, its *parent* widget
   is passed as the first argument to the widget constructor.

configuration options
   Widgets have *configuration options*, which modify their appearance
   and behavior, such as the text to display in a label or button.
   Different classes of widgets will have different sets of options.

geometry management
   Widgets aren't automatically added to the user interface when they
   are created. A *geometry manager* like "grid" controls where in the
   user interface they are placed.

event loop
   Tkinter reacts to user input, changes from your program, and even
   refreshes the display only when actively running an *event loop*.
   If your program isn't running the event loop, your user interface
   won't update.


Understanding How Tkinter Wraps Tcl/Tk
--------------------------------------

When your application uses Tkinter's classes and methods, internally
Tkinter is assembling strings representing Tcl/Tk commands, and
executing those commands in the Tcl interpreter attached to your
applicaton's "Tk" instance.

Whether it's trying to navigate reference documentation, trying to
find the right method or option, adapting some existing code, or
debugging your Tkinter application, there are times that it will be
useful to understand what those underlying Tcl/Tk commands look like.

To illustrate, here is the Tcl/Tk equivalent of the main part of the
Tkinter script above.

   ttk::frame .frm -padding 10
   grid .frm
   grid [ttk::label .frm.lbl -text "Hello World!"] -column 0 -row 0
   grid [ttk::button .frm.btn -text "Quit" -command "destroy ."] -column 1 -row 0

Tcl's syntax is similar to many shell languages, where the first word
is the command to be executed, with arguments to that command
following it, separated by spaces. Without getting into too many
details, notice the following:

* The commands used to create widgets (like "ttk::frame") correspond
  to widget classes in Tkinter.

* Tcl widget options (like "-text") correspond to keyword arguments in
  Tkinter.

* Widgets are referred to by a *pathname* in Tcl (like ".frm.btn"),
  whereas Tkinter doesn't use names but object references.

* A widget's place in the widget hierarchy is encoded in its
  (hierarchical) pathname, which uses a "." (dot) as a path separator.
  The pathname for the root window is just "." (dot). In Tkinter, the
  hierarchy is defined not by pathname but by specifying the parent
  widget when creating each child widget.

* Operations which are implemented as separate *commands* in Tcl (like
  "grid" or "destroy") are represented as *methods* on Tkinter widget
  objects. As you'll see shortly, at other times Tcl uses what appear
  to be method calls on widget objects, which more closely mirror what
  would is used in Tkinter.


How do I...? What option does...?
---------------------------------

If you're not sure how to do something in Tkinter, and you can't
immediately find it in the tutorial or reference documentation you're
using, there are a few strategies that can be helpful.

First, remember that the details of how individual widgets work may
vary across different versions of both Tkinter and Tcl/Tk. If you're
searching documentation, make sure it corresponds to the Python and
Tcl/Tk versions installed on your system.

When searching for how to use an API, it helps to know the exact name
of the class, option, or method that you're using. Introspection,
either in an interactive Python shell or with "print()", can help you
identify what you need.

To find out what configuration options are available on any widget,
call its "configure()" method, which returns a dictionary containing a
variety of information about each object, including its default and
current values. Use "keys()" to get just the names of each option.

   btn = ttk.Button(frm, ...)
   print(btn.configure().keys())

As most widgets have many configuration options in common, it can be
useful to find out which are specific to a particular widget class.
Comparing the list of options to that of a simpler widget, like a
frame, is one way to do that.

   print(set(btn.configure().keys()) - set(frm.configure().keys()))

Similarly, you can find the available methods for a widget object
using the standard "dir()" function. If you try it, you'll see there
are over 200 common widget methods, so again identifying those
specific to a widget class is helpful.

   print(dir(btn))
   print(set(dir(btn)) - set(dir(frm)))


Navigating the Tcl/Tk Reference Manual
--------------------------------------

As noted, the official Tk commands reference manual (man pages) is
often the most accurate description of what specific operations on
widgets do. Even when you know the name of the option or method that
you need, you may still have a few places to look.

While all operations in Tkinter are implemented as method calls on
widget objects, you've seen that many Tcl/Tk operations appear as
commands that take a widget pathname as its first parameter, followed
by optional parameters, e.g.

   destroy .
   grid .frm.btn -column 0 -row 0

Others, however, look more like methods called on a widget object (in
fact, when you create a widget in Tcl/Tk, it creates a Tcl command
with the name of the widget pathname, with the first parameter to that
command being the name of a method to call).

   .frm.btn invoke
   .frm.lbl configure -text "Goodbye"

In the official Tcl/Tk reference documentation, you'll find most
operations that look like method calls on the man page for a specific
widget (e.g., you'll find the "invoke()" method on the ttk::button man
page), while functions that take a widget as a parameter often have
their own man page (e.g., grid).

You'll find many common options and methods in the options or
ttk::widget man pages, while others are found in the man page for a
specific widget class.

You'll also find that many Tkinter methods have compound names, e.g.,
"winfo_x()", "winfo_height()", "winfo_viewable()". You'd find
documentation for all of these in the winfo man page.

Note:

  Somewhat confusingly, there are also methods on all Tkinter widgets
  that don't actually operate on the widget, but operate at a global
  scope, independent of any widget. Examples are methods for accessing
  the clipboard or the system bell. (They happen to be implemented as
  methods in the base "Widget" class that all Tkinter widgets inherit
  from).


Threading model
===============

Python and Tcl/Tk have very different threading models, which
"tkinter" tries to bridge. If you use threads, you may need to be
aware of this.

A Python interpreter may have many threads associated with it. In Tcl,
multiple threads can be created, but each thread has a separate Tcl
interpreter instance associated with it. Threads can also create more
than one interpreter instance, though each interpreter instance can be
used only by the one thread that created it.

Each "Tk" object created by "tkinter" contains a Tcl interpreter. It
also keeps track of which thread created that interpreter. Calls to
"tkinter" can be made from any Python thread. Internally, if a call
comes from a thread other than the one that created the "Tk" object,
an event is posted to the interpreter's event queue, and when
executed, the result is returned to the calling Python thread.

Tcl/Tk applications are normally event-driven, meaning that after
initialization, the interpreter runs an event loop (i.e.
"Tk.mainloop()") and responds to events. Because it is single-
threaded, event handlers must respond quickly, otherwise they will
block other events from being processed. To avoid this, any long-
running computations should not run in an event handler, but are
either broken into smaller pieces using timers, or run in another
thread. This is different from many GUI toolkits where the GUI runs in
a completely separate thread from all application code including event
handlers.

If the Tcl interpreter is not running the event loop and processing
events, any "tkinter" calls made from threads other than the one
running the Tcl interpreter will fail.

A number of special cases exist:

   * Tcl/Tk libraries can be built so they are not thread-aware. In
     this case, "tkinter" calls the library from the originating
     Python thread, even if this is different than the thread that
     created the Tcl interpreter. A global lock ensures only one call
     occurs at a time.

   * While "tkinter" allows you to create more than one instance of a
     "Tk" object (with its own interpreter), all interpreters that are
     part of the same thread share a common event queue, which gets
     ugly fast. In practice, don't create more than one instance of
     "Tk" at a time. Otherwise, it's best to create them in separate
     threads and ensure you're running a thread-aware Tcl/Tk build.

   * Blocking event handlers are not the only way to prevent the Tcl
     interpreter from reentering the event loop. It is even possible
     to run multiple nested event loops or abandon the event loop
     entirely. If you're doing anything tricky when it comes to events
     or threads, be aware of these possibilities.

   * There are a few select "tkinter" functions that presently work
     only when called from the thread that created the Tcl
     interpreter.


Handy Reference
===============


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 *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 "config()"
method without arguments, or by calling the "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 lookup  | "'Relief'"     |
+---------+-----------------------------------+----------------+
| 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
----------

The packer is one of Tk's geometry-management mechanisms.    Geometry
managers are used to specify the relative 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 "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 "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 "tkinter".

There are many useful subclasses of Variable already defined:
"StringVar", "IntVar", "DoubleVar", and "BooleanVar".  To read the
current value of such a variable, call the "get()" method on it, and
to change its value you call the "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:

   import tkinter as tk

   class App(tk.Frame):
       def __init__(self, master):
           super().__init__(master)
           self.pack()

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

           # Create the application variable.
           self.contents = tk.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

           # Define a callback for when the user hits return.
           # It prints the current value of the variable.
           self.entrythingy.bind('<Key-Return>',
                                self.print_contents)

       def print_contents(self, event):
           print("Hi. The current entry content is:",
                 self.contents.get())

   root = tk.Tk()
   myapp = App(root)
   myapp.mainloop()


The Window Manager
------------------

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
"tkinter", these commands have been implemented as methods on the "Wm"
class.  Toplevel widgets are subclassed from the "Wm" class, and so
can call the "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 "_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:

   import tkinter as tk

   class App(tk.Frame):
       def __init__(self, master=None):
           super().__init__(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
--------------------

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 "cursorfont.h" can be used,
   without the "XC_" prefix.  For example to get a hand cursor
   ("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 "set()" method of some scrollbar widget,
   but can be any widget method that takes a single argument.

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


Bindings and Events
-------------------

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

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

Notice how the widget field of the event is being accessed in the
"turn_red()" 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 "tkinter" functions to
   access these special points in text widgets:

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 "tkinter.Image":

* "BitmapImage" for images in XBM format.

* "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.

See also:

  The Pillow package adds support for formats such as BMP, JPEG, TIFF,
  and WebP, among others.


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 "BufferedIOBase" or "TextIOBase" "read()" or
"readline()" methods, since these will insist on reading a predefined
number of bytes. For sockets, the "recv()" or "recvfrom()" methods
will work fine; for other files, use raw reads or
"os.read(file.fileno(), maxbytecount)".

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

   Registers the file handler callback function *func*. The *file*
   argument may either be an object with a "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)

Widget.tk.deletefilehandler(file)

   Unregisters a file handler.

tkinter.READABLE
tkinter.WRITABLE
tkinter.EXCEPTION

   Constants used in the *mask* arguments.
