import media
# This program contrasts assignment when immutable and mutable objects are involved

# First is an example with immutable objects (ints, here)

a = 19

b = a

a = 42

print a,b  # prints 42 19
print "As you saw, this prints 42 and 19 - a and b associated with different objects"

# here's an analogous example using mutable objects

file_name = media.choose_file()
a = media.load_picture(file_name)

b = a

for pixel in a:
    media.set_green(pixel,0)
    
raw_input("enter 1 when you want to show a ")
media.show(a)

raw_input("enter 1 when you want to show b ")
media.show(b)

print "As you saw, a and b point to the same object - so you saw the same picture twice"
# You will see exactly the same picture!

# what if we want to display before and after pictures?  You'll need
# to have two copies of the picture.  

raw_input("enter 1 when you want to go on to the next example ")
file_name = media.choose_file()
a = media.load_picture(file_name)
b = media.load_picture(file_name) # this replaces b = a

for pixel in a:
    media.set_green(pixel,0)
 
raw_input("when you want to see a, the 'after' picture, type 1 ")    
media.show(a) # The "after" picture
raw_input("when <you want to see b, the 'before' picture, type 1 ")
media.show(b) # The "before" picture
print "first you saw the 'after' picture, then you saw the 'before' picture"