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Tkinter Lesson 3 Notes: Using bind() To Make it Work
Concepts
- To add functionality to a GUI you must:
- Define an initializing event
- What did the user do?
- press a key, click a button, move the mouse
- Define a function
- What should the program do in response?
- execute a function, launch a new window, end program
- Associate the event and function with part of the GUI
- To which widget does this behavior apply?
- Does this feature apply to the whole window or application?
bind()
- The bind() method ties together a widget, a user action, and an event
- The bind() method takes two arguments:
- sequence (the user action that triggers an event)
- function (the function to run when triggered)
- A function must be defined before it is called by bind()
Sample bind()
from Tkinter import *
root = Tk()
btn = Button(root, text = "End")
btn.grid()
def stop(event):
root.destroy()
btn.bind('<Button-1>', stop)
- When the button is clicked, the window is destroyed.
- Notice that the function stop() takes "event" as an argument. The event is some data passed automatically from bind to the function. If you do not list it as a parameter, the function will crash.
- The first argument in bind() is a string in quotation marks.
- Note that the function stop is called without parenthesis
Part 1: What are you binding to?
- What to bind() with
- Functions can be bound to a widget or to a whole window.
- If a function is specific to a widget, bind it to the widget
- Button is pressed, function is called
- [Enter] button pressed while user is typing in Entry box, function is called
- If a function applies to the window as a whole, bind it to the root window
- Pressing [F1] brings up a help menu
- Pressing [Delete] destroys the window
- Binding Example
from Tkinter import *
root = Tk()
Label(root, text = "Deadly Box").grid()
def stop(event):
root.destroy()
box = Entry(root, width=20)
box.grid()
root.bind('9', stop)
box.bind('X', stop)
- If the user types "X" while in the Entry box, the window will close. Hitting "X" at any other time will have no effect.
- If the user presses the "9" key at any time, the window will close.
Part 2: What event are you watching for?
Event Types
- Events are the things that happen and trigger a response. They are the first argument in a bind() call.
btn.bind('event', function)
- There are three basic types of events that bind() can watch out for:
- mouse events (click, mouse enters window, mouse moves)
- keyboard events (key pressed, key combination pressed)
- Environment changes (window resized, image changes, etc.)
- To bind a function with an event, you must use a "sequence" keyword, or a combination of sequence keywords.
Some Sequence Event Keywords
- Mouse Sequences
<Enter> -Mouse enters field of widget
<Leave> -Mouse leaves field of widget
<Button-1> -Left mouse button clicked
<Button-3> -Right mouse button
<Double-Button-1> - Double click left mouse button
<B1-Motion> -Mouse moved while left button is held down.
<ButtonRelease> - Mouse button has been let go.
- example:
btn.bind('<Button-1>', stop)
- Keyboard Sequences
<Key> -any key has been pressed
<KeyRelease> -any key has been released
<Return> - [Enter] key pressed
X -A capital "X" pressed.
- example:
root.bind('<Return>', func)
root.bind('R', func)
Anatomy of a Event Sequence
- Event sequences take the form <Event-modifier>
- Event sequence combinations are created by adding a hyphen and a new keyword.
<Control-Shift-space>
<Shift-Button-1>
- The keywords are case sensitive
- Alphabet letters can by indicated without < >, everything else needs to be inside < >
Keyboard Event Reference
BackSpace
Break
Control_L
Delete
Down
End
Escape
F1, F2 …
Home
Insert
Left
Num_Lock
Print
Return
Right
Shift_L
Shift_R
Tab
Up
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backslash
bracketleft
bracketright
colon
comma
dollar
equal
exclam
minus
numbersign
percent
period
plus
slash
space
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Keypad Buttons
KP_1 , KP_2 …
KP_Add
KP_Decimal
KP_Divide
KP_Enter
KP_Equal
KP_Multiply
KP_Subtract
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Special Event Sample
from Tkinter import *
root = Tk()
Label(root, text = "Deadly Box").grid()
def stop(event):
root.destroy()
box = Entry(root, width=20)
box.grid()
root.bind('<Control-F10>', stop)
root.bind('<Shift-space>', stop)
- Two special combinations are set up. If the user holds down shift and presses the space bar, or if he holds control and presses F10, the window will disappear.
Part 3: What does the program do when triggered?
- Generally Available Methods
widget.configure()
This will change whatever configuration options you specify. The changes will automatically appear on the screen.
widget.destroy()
This will destroy the widget and remove it from the GUI.
- These methods can be used on most widgets
- .configure() Example
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import random
from Tkinter import *
root = Tk()
def colorchange(event):
colors = ['red', 'blue', 'green', 'yellow', 'orange']
color = random.choice(colors)
btn.configure(bg=color)
btn = Button(root, text= "Chameleon")
btn.grid()
btn.bind('<Button-1>', colorchange)
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- The button is bound to the colorchange function. This function randomly picks a color and uses btn.configure() to apply this color to the button background.
- The .configure() method can be used on any option of a widget.
- Entry and Text Methods
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widget.get(index1, index2)
Gets the text currently in the Entry or Text widget. Can specify a start and end index number, or leave blank to get everything.
widget.insert(index1, text)
Insert text into the widget at the given index point.
widget.delete(index1, index2)
Delete the characters from index1 to index2. This method requires both indexes.
- Text Method Examples
from Tkinter import *
root = Tk()
lbl = Label(root, text = "Your Text Here", bg = 'blue', fg= 'white')
lbl.grid()
textbox = Entry(root, width="20")
textbox.grid()
def lblupdate(event):
newtext = textbox.get()
textbox.delete(0, len(newtext))
lbl.configure(text = newtext)
textbox.bind('<Return>', lblupdate)
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