title: 'self' parameter in Python non-class functions
body: <p>I'm currently teaching some basic Python to an eager primary school student, and I'm having a bit of trouble wrapping my head around a strange little anomaly in this code for an etch-a-sketch.</p>
  
  <pre><code># myEtchASketch application
  
  from tkinter import *
  
  #####Set variables
  canvas_height = 400
  canvas_width=600
  canvas_colour="black"
  
  p1_x=canvas_width/2
  p1_y=canvas_height
  p1_colour="blue"
  line_width=6
  line_length=9
  
  ####New window
  window=Tk()
  window.title("MyEtchASketch")
  canvas=Canvas(bg=canvas_colour, height=canvas_height, width=canvas_width,highlightthickness=0)
  canvas.pack()
  
  ##### Functions:
  
  #player controls
  def p1_move_N(self):
      global p1_y
      canvas.create_line(p1_x, p1_y, p1_x, (p1_y-line_length), width=line_width, fill=p1_colour)
      p1_y = p1_y - line_length
  
  def p1_move_S(self):
      global p1_y
      canvas.create_line(p1_x, p1_y, p1_x, (p1_y+line_length), width=line_width, fill=p1_colour)
      p1_y=p1_y+line_length
  
  def p1_move_W(self):
      global p1_x
      canvas.create_line(p1_x, p1_y, (p1_x-line_length), p1_y, width=line_width, fill=p1_colour)
      p1_x=p1_x-line_length
  
  def p1_move_E(self):
      global p1_x
      canvas.create_line(p1_x, p1_y, (p1_x+line_length), p1_y, width=line_width, fill=p1_colour)
      p1_x=p1_x+line_length
  
  def erase_all(self):
      canvas.delete(ALL)
  
  
  #bind movement to key presses
  window.bind("&lt;Up&gt;", p1_move_N)
  window.bind("&lt;Down&gt;",p1_move_S)
  window.bind("&lt;Left&gt;", p1_move_W)
  window.bind("&lt;Right&gt;", p1_move_E)
  window.bind("&lt;u&gt;", erase_all)
  
  window.mainloop()
  </code></pre>
  
  <p>The player controls are what's bugging me. The code doesn't work if I don't have <code>self</code> added. I get:</p>
  
  <pre><code>Exception in Tkinter callback
  Traceback (most recent call last):
    File "C:\Users\ThisIsNotMyRealUserFolder\AppData\Local\Programs\Python\Python35-32\lib\tkinter\__init__.py", line 1550, in __call__
      return self.func(*args)
  TypeError: p1_move_N() takes 0 positional arguments but 1 was given
  </code></pre>
  
  <p>I've done a bit of research and found that <code>self</code> is used for class methods. As you can see, none of the functions are class methods. The only thing I can think of is that <code>bind()</code> treats all functions passed to it as class methods, meaning they require <code>self</code> to function correctly.</p>
  
  <p>Any and all help is appreciated.</p>