CODEWITHSUNDEEP
python-2.7tkinter
DrBwts
DrBwts

Reputation: 3657

Tkinter Python objects blank canvas?

I am learning tkinter as well as trying to get to grips with classes & objects.

I have created and object that draws a line from a given coordinate to where ever the mouse is clicked. Well that's what I thought I had done but what I get is just a blank canvas.

My code,

from Tkinter import *

class App:

    def __init__(self, master):

        frame = Frame(master)
        frame.pack()

        self.can = Canvas(master, width=200, height=100)
        self.can.configure(cursor="crosshair")
        self.can.pack()
        self.start_point = [50, 50]
        self.end_point = None
        self.can.bind("<Button-1>", lambda ev: self.line_end)

    def line_end(self, event):      
        self.can.create_line(self.start_point[0], self.start_point[1], event.x, event.y)

root = Tk()
app = App(root)
root.mainloop()

Is there some kind of push function like openGL? If so I cant find it.

Upvotes: 0

Views: 1073

Answers (1)

Bryan Oakley
Bryan Oakley

Reputation: 385900

The problem is that your binding is not calling your function. You can put a print statement inside your function to prove that.

The reason is that you're using a lambda like this:

self.can.bind("<Button-1>", lambda ev: self.line_end)

Within the body of the lambda you must actually call the function, not just reference it. To do that, you call it like you do anywhere else: with parenthesis on the end:

self.can.bind("<Button-1>", lambda ev: self.line_end(ev))

However, you don't need lambda in this instance. Normally you should just give the command attribute a reference to a function, and not use lambda at all. Lambda is only necessary if you want to pass some additional parameters to your function, which is often not necessary in event-driven programming.

Change your binding to this:

self.can.bind("<Button-1>", self.line_end)

Upvotes: 2

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