OO and game design questions

Jonathan Hartley tartley at tartley.com
Wed Oct 20 12:33:30 CEST 2010

On Oct 20, 11:25 am, Jonathan Hartley <tart... at tartley.com> wrote:
> On Oct 18, 8:28 am, dex <josipmisko... at gmail.com> wrote:
> > I'm building a turn based RPG game as a hobby. The design is becoming
> > increasingly complicated and confusing, and I think I may have
> > tendency to over-engineer simple things. Can anybody please check my
> > problems-solutions and point me to more elegant solution?
> > Every item/character/room is a separate object. Items/characters need
> > to have references to room they are in, and room needs to have a list
> > of references to items/characters that are contained within. I decided
> > to use weak references. That way I can destroy object by deleting it,
> > I don't have to destroy all references as well. In each object's
> > __init__() that object is added to game_object list, and in each
> > __del__() they are removed from game_object list. This mechanism keeps
> > them safe from garbage collector. How pythonic is this design?
> > In turn-based games, the order of action execution in battle can give
> > unfair advantage to players. For example, if player's arm is crippled
> > before his action is executed, he would do less damage. To offset
> > this, I first execute all players' actions and calculate effects in
> > first pass, then apply the effects in second pass. The effect can be
> > health decrease by 15HP, item pick-up, 30p experience gain, etc. This
> > means the player deals the same amount of damage no matter what
> > happens to him in that turn. The difficult part is keeping track of
> > various effects. I had to make separate class for various types of
> > effects (ChangeAttributeEffect, GetItemEffect, LooseItemEffect). Each
> > class stores weak reference to target object and has apply() method
> > that applies the effect to object. I'm not satisfied with this as it's
> > limiting, error-prone and uses metaprogramming. Is there a design
> > pattern that would remember changes to an object, and apply them
> > later?
> > Sorry for the wall of text.
> One common way to store delayed actions is as a lambda (an anonymous
> function.) A lambda defines a new function:
> , and you can call this function later. The created function has no
> name, (but you can assign it to a variable to give it a name if you
> like) and can be called later:
> So in the game, you could have a collection 'effects', each one will
> be a lambda:
>   effects = []
> At the start of the round, as each entity makes its moves, they add
> lambdas to this collection.
>   effects.append(
>       lambda: decrease_hp(monster_a, 4)
>   )
>   effects.append(
>       lambda: lose_item(monster_a, item_b)
>   )
> Instead of appending it directly like this, I imagine the lambdas
> could be returned by the monster's 'act' or 'update' method:
>   class Monster():
>     def act(self):
>       # blah and finally
>       return lambda: decrease_hp(monster_a, 4)
> Then for the start of a round, first you ask each monster what action
> it is going to perform:
>   for monster in room.monsters:
>       effects.append(
>           monster.act()
>       )
> Then for the end of the round, call all the lambdas
>   for effect in effects:
>       effect()

Also, I second other people's suggestions that you almost never need
to be using __del__ nor weak references in Python, unless you are
writing a project that is specifically related to complex resource
allocation issues. In an rpg game like this, just store references to
the objects that you need (which you have to do anyway, to use them),
and forget about allocation issues.

Don't be afraid to post follow-up questions, (or even to mail me off
list.) I make hobbyist OpenGL games in Python myself, and although I'm
no expert, I'd love to chat more about this for our mutual benefit.

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