Unable to update TKinter graph - user-interface

Question

Colleagues,
I am designing a GUI with two buttons and one is to display a graph, hourly temperature.
The issue that I am facing is that I can not make a function(update_graph) that updates the value with self.after.
This part creates page 1 and i working fine, until I call update_graph
class PageOne(tk.Frame):
def __init__(self, parent, controller):
tk.Frame.__init__(self, parent)
label = tk.Label(self, text="Page One!!!", font=LARGE_FONT)
label.pack(pady=10,padx=10)
button1 = tk.Button(self, text="Back to Home",
command=lambda: controller.show_frame(StartPage))
button1.pack()
button2 = tk.Button(self, text="Page Two",
command=lambda: controller.show_frame(PageTwo))
button2.pack()
canvas = Canvas(self, width=400, height=400, bg = 'white')
canvas.pack()
# create x and y axes
canvas.create_line(100,250,400,250, width=2)
canvas.create_line(100,250,100,50, width=2)
# creates divisions for each axle
for i in range(11):
x = 100 + (i * 30)
canvas.create_line(x,250,x,245, width=2)
canvas.create_text(x,254, text='%d'% (10*i), anchor=N)
for i in range(6):
y = 250 - (i * 40)
canvas.create_line(100,y,105,y, width=2)
canvas.create_text(96,y, text='%5.1f'% (50.*i), anchor=E)
self.update_graph()
def update_graph(self):
# here is canvas create line that causes a trouble
canvas.create_line(100,250,140,200, width=2)
self.after(100,self.update_graph)
Whith this code I get an error "canvas is not defined".
If I add self to canvas in update_graph, I get
self.canvas.create_line(100,250,140,200, width=2)
AttributeError: 'PageOne' object has no attribute 'canvas'
What am I missing here?

Answer 1

canvas is only defined in the scope of the constructor (__init__) method. If you want to be able to access it elsewhere in the class, you need to make it an instance variable. Instead of,
canvas = Canvas(self, width=400, height=400, bg = 'white')
make it,
self.canvas = Canvas(self, width=400, height=400, bg = 'white')
now, everywhere else in the code where you reference canvas, change it to self.canvas. This should fix the problem.
On an unrelated note, a problem that I'm seeing in update_graph is that it calls itself recursively, or over and over. Perhaps you could change it to something like this:
def update_graph(self):
# This line is quite long. Maybe you could shorten it?
self.after(100, lambda: canvas.create_line(100,250,
140,200, width=2))
Hope this helps!
EDIT: My redefinition of update_graph only makes sense if you want one fixed line drawn. If you intend to add other functionality, such as periodic updates, the original code is correct, as Bryan pointed out.