Why a 300*300 Oval cannot fit a 300*300 Canvas perfectly?

Question

from Tkinter import *


class Ball:
    def __init__(self, canvas, x1, y1, x2, y2):
        self.x1 = x1
        self.y1 = y1
        self.x2 = x2
        self.y2 = y2
        self.canvas = canvas
        self.ball = canvas.create_oval(self.x1, self.y1, self.x2, self.y2, fill="red")

# initialize root Window and canvas
root = Tk()
root.title("Balls")
root.resizable(False, False)
canvas = Canvas(root, width=300, height=300)
canvas.pack()

# create ball objects
ball = Ball(canvas, 0, 0, 300, 300)
root.mainloop()

As you see, I draw a 300*300 Oval in a 300*300 canvas, but the oval didn't fit the canvas perfectly. The result is(Mac OS):

So my question is:

1. Why it's not fit perfectly?
2. What should I do to avoid such awkward things?

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EDIT:

ball = Ball(canvas, 1, 1, 299, 299) doesn't work either, it gives:

Answer 1

The drawable area of the canvas is by default less than the total width and height of the canvas. The total size includes the widget border and the highlight ring. If you want the drawable area to be 100% of the width and height then you need to set both the borderwidth and highlightthickness attributes of the canvas to 0 (zero).