Introductory Programming
Fall 2004

Python Objects
--------------

Big ideas:

0) the most basic use of objects is to collect related data
   in a single unit

   Example: a Point object contains coordinates

1) defining a new class defines a new type of object

2) then you can create new objects with the new type

 
Try the following tests in a Python interpreter:

1) Create a new class

>>> class Point: pass

Creating a class creates a "class object"

>>> print Point
__main__.Point
>>> print type(Point)
<type 'classobj'>


2) Use the class object to create instances of the class.

>>> p = Point()

p now refers to a Point object

>>> print p
<__main__.Point instance at 0xf6f7678c>
>>> print type(p)
<type 'instance'>


Common error: what happens if you forget the () ?

>>> p = Point
>>> print p


3) Add attributes to an object

>>> p.x = 100
>>> p.y = 200
>>> print p.x, p.y
100 200

The Point named p contains two pieces of data, named x and y.

What does the state diagram look like?


4) More than one variable can refer to the same object

>>> q = p

What does the state diagram look like now?

If you change "q", you are also changing "p", because
q and p are aliases for the same object.

>>> q.x = 300
>>> print p.x, p.y
300 200

5) How do we know that p and q are the same object?

>>> id(p)
-151558260
>>> id(q)
-151558260


6) The copy module provides functions for copying objects:

>>> import copy
>>> r = copy.copy(q)


7) The is operator tells you whether two variables refer to
   the same object

>>> p is q
>>> p is r
>>> q is r


8) For user-defined classes, the == operator is the same as is.

>>> p == q
>>> p == r
>>> q == r

But we will revisit this operator later!


Amoeba redux
------------

Type the following program into a file named amoeba.py

from World import *
from time import sleep

world = AmoebaWorld()
amy = Amoeba(world)

for i in range(10):
    amy.redraw(i, i)
    world.update()
    sleep(0.5)

world.mainloop()


So you can move Amoebas around by invoking redraw with the new
coordinates, but in general, Amoebas don't know where they are!

   (notice that redraw is a method you invoke on an Amoeba)

Add code so that each time the Amoeba moves, it stores the
new x and y coordinates as attributes of the Amoeba.

Now check out the teleport function in ip_hw05_soln.py

def teleport(a, x, y):
    # move amoeba a to the given location
    a.x = x
    a.y = y
    a.redraw(a.x, a.y)

A couple of things happening here:

1) you can pass objects as parameters

2) the parameter is a temporary alias for the object

3) if you change the object in a function, the object stays changed

   (unlike local variables, which disappear when the function ends)

One more time, draw a state diagram that shows this effect.