Question

One question: How do you dynamically draw to a Tkinter Canvas object from a button click?

This topic was posted originally by user339860 (see 1) as a two part question, but the second portion of the question has not yet been addressed. I’m experiencing the same problem, specifically I cannot draw to a Canvas object from a button event. Getting this question answered will help two people, myself and user339860; please take a look.

The application creates two frames positioned left and right, the second frame contains the Canvas object. I have a button in the first frame bound to a function/method named drawRectangle. The application runs fine, it even draws a rectangle in the Canvas object using the create_rectangle method, until you click the button. When you click the button you get a message with the following;

tkinter_app_27Nov2010.py", line 25, in drawRectangle
self.myCan.create_rectangle(64,64,110,110,fill=’blue’)
AttributeError: ‘NoneType’ object has no attribute ‘create_rectangle’

I thought it might have something to do with the scope of the Canvas object, so I created a class level variable set to None, but that didn’t fix the issue. I thought about the Canvas display list (see 2), but the Tk manual pages don’t reflect a method of adding a new object that I could find.

CODE:

---

# tkinter_app_27Nov2010.py
from Tkinter import *
class Application(Frame):
myCan = None
def createWidgets(self):
    uiFrame = Frame(self,width=300,height=30)
    uiFrame.configure(background='#e0e0e0')
    uiFrame.grid(row=0,column=0,sticky=N+S)

    outputFrame = Frame(self,width=300,height=300,background='#C0C0C0')
    outputFrame.grid(row=0,column=1)

    newBtn = Button(uiFrame,text="testing",command=self.drawRectangle)
    newBtn.grid(row=0,column=0)
    fillLbl = Label(uiFrame,text='-').grid(row=1,sticky=N+S)
    
    newLBL = Label(outputFrame,text="another testing",background='#C0C0C0')
    newLBL.grid(row=0)

    myCan = Canvas(outputFrame,width=300,height=300,borderwidth=1,relief='sunken')
    myCan.grid(row=1)
    myCan.create_rectangle(34,34,50,50,fill='red')

def drawRectangle(self):
    self.myCan.create_rectangle(64,64,110,110,fill='blue')
    
def __init__(self,master):
    Frame.__init__(self,master)
    self.pack()
    self.createWidgets()

root = Tk()
myApp = Application(master=root)
root.title("Tkinter Testing!")
myApp.mainloop()

---

There has to be a way to get a handle on the "damage/repair display model" (see 3) the Tkinter Canvas object uses to update itself. Please help!

References:

1. stackoverflow.com/questions/2824041/dynamically-add-items-to-tkinter-canvas

2. http://www.tcl.tk/man/tcl8.4/TkCmd/canvas.htm#M16

3. effbot.org/tkinterbook/canvas.htm#performance-issues

Answer 1

This is a Python problem, not a tkinter one. You have defined local variables inside createWidgets, but you haven’t set them as instance attributes. You need to use self.foo for that:

>>> class Foo:
...     def __init__(self):
...             bar = "baz"
...
>>> class Bar:
...     def __init__(self):
...             self.bar = "baz"
...
>>> foo = Foo()
>>> foo.bar
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: Foo instance has no attribute 'bar'
>>> bar = Bar()
>>> bar.bar
'baz'

Notice that you are indeed correct: the problem is to do with the scope of the Canvas. Or more precisely, with the scope of the myCan variable. If you hadn’t defined the class variable myCan, the lookup self.myCan would have raised a revealing AttributeError.