[Tutor] confusion about cloning (graphics)

[David]

Hello list,

I continue my reading of Harrington. In section 2.4. he is making a 
point about the need to clone, as otherwise the object associated to 
parameter corner (the point (20, 50)) takes the same value as corner2. 
That is: corner2 changes corner, which in turn changes the point (20, 
50). Here is the faulty code Harrington provides:

<code>

'''Program: makeRectBad.py
Attempt a function makeRect (incorrectly),
which takes a takes a corner point and dimensions to construct a Rectangle.
'''

from graphics import * # based on Zelle's graphics.py module

def makeRect(corner, width, height):  # Incorrect!
     '''Return a new Rectangle given one corner Point and the dimensions.'''
     corner2 = corner
     corner2.move(width, height)
     return Rectangle(corner, corner2)

def main():
     winWidth = 300
     winHeight = 300
     win = GraphWin('Draw a Rectangle (NOT!)', winWidth, winHeight)
     win.setCoords(0, 0, winWidth, winHeight)

     rect = makeRect(Point(20, 50), 250, 200)
     rect.draw(win)

     # Wait for another click to exit
     msg = Text(Point(winWidth/2, 20),'Click anywhere to quit.')
     msg.draw(win)
     win.getMouse()
     win.close()

main()

</code>

This behaviour stuns me, because I was always following the belief that 
when variable2 refers to another variable1, the value of variable1 would 
NOT change, even as I operate on variable2 - just like my little 
experiment at the promt:

In [10]: x = 5

In [11]: y = x

In [12]: y + 2
Out[12]: 7

In [13]: x
Out[13]: 5

So here is my question: what have I failed to grasp? Are those different 
issues? If so, why?

I'd appreciate it if you could give me a short comment, this one bothers 
me ;-)

David