CSCI 171 Questions and Answers

PyGame

• Can we look at an asteroids-style example in pygame?
• How do I let the player set direction using mouse clicks?

  Python

• How can I get 2D arrays in python?
• How are destructors handled in Python? (the counterpart to Python's __init__ constructor)

  Sample PyGame files

• Simple game display with an image
• Simple controls and movement
• An asteroids-style spaceship
• A typical pygame file layout

  Sample Python files

• Arrays and 2D arrays in Python

PyGame

Can we look at an asteroids-style example in pygame?

I actually ran across a good introductory one on the web, 
   so will simply post a link for now, from
    Comp Sci at UNSW 

How do I let the player set direction using mouse clicks?

To determine the direction from one object's position to another in PyGame, here's a direction function using a quick and dirty bit of trig. You can use pygame.mouse.get_pos() to look up the current position of the mouse and pass that as x2,y2.

 # compute the direction from (x1,y1) to (x2,y2)
 # assuming 0-360 degrees goes counterclockwise 
 #    with 0 degrees facing directly to the right
 def comp_direction(x1, y1, x2, y2):

     # handle a few special cases
     if (x1 == x2):
         if (y1 < y2): return 0
	 else: return 180
     elif (y1 == y2):
         if (x1 < x2): return 90
	 else: return 270

     # compute the degrees in radians, using tan = opp/adj
     #    and arctan(tan) to get the actual angle
     rad_tan = (x2 - x1) / (y2 - y1)
     rad_angle = math.atan(rad_tan)

     # now convert the angle to degrees 
     deg_angle = 180 * rad_angle / math.pi

     # note that deg_angle will be in the range -90 to +90,
     # and we have to convert to a 0-360 degree value based
     # on which quadrant (x2,y2) falls into relative to (x1,y1)
     #                   |
     #   deg_angle < 0   | deg_angle > 0
     #   we want 90-180  | we want 0-90
     #                   |
     #   ----------------+----------------
     #                   |
     #   deg_angle > 0   | deg_angle < 0
     #   we want 180-270 | we want 270-360
     #                   |
     if (x1 < x2):
         if (y1 < y2): return deg_angle
	 else: return 360 + deg_angle
     elif (y1 < y2): return 180 + deg_angle
     else: return 180 + deg_angle
 

Python

How can I get 2D arrays in python?

# you can use a list of lists to act like a two-dimensional array
arr2d = [ [0, 1], [2, 3], [4, 5] ]
print arr2d

# to access an entire row, refer to it by the row index, e.g.: 
print arr2d[0]
# to access a single element, use the [row][column] style, e.g.:
print arr2d[0][0]

# add a new row to the array (i.e. append a new element to the list) use the append method:
arr.append([6,7])
print arr2d

# similarly, to add a new element to the end of a row, apply append to a specific row, e.g:
arr2d[0].append(-1)
print arr2d

Python's __init__ method is run on object creation, but which method is run on object destruction?

   # __del__ is the Python equivalent of a destructor, e.g.
   class myObj:
      def __init__(self):
          print "object created"

      def __del__(self):
          print "object destroyed"

   myvar =  x()
   del x
   

Just as a general note, local variables will automatically be destroyed when their scope ends (e.g. when a function ends all of its local variables get destroyed then). For other objects, Python periodically runs a garbage collector to find/destroy objects that are no longer referenced/accessible.

Language trivia: sometimes developers create objects that reference themselves, or cyclic references where A references B and B references A. In such cases, the order they get destroyed in would be arbitrary, and Python won't destroy them if you have created your own __del__ method for the objects.