Pass `special_flags` argument to group.draw in pygame

Question

Is there a way to pass the special_flags argument to Group.draw so that it calls the .blit method with those flags? I've tried just passing it as a keyword argument like this:

group.draw(surface, special_flags=pygame.BLEND_SOURCE_ALPHA)

but it gives this error:

Traceback (most recent call last):
  File "C:\Users\MarciAdam\PycharmProjects\pygame_stuff_1\main.py", line 394, in <module>
    group.draw(surface, special_flags=pygame.BLEND_RGBA_MAX)
TypeError: draw() got an unexpected keyword argument 'special_flags'

I know I could do something like this:

for sprite in group.sprites():
    surface.blit(sprite.image, sprite.rect, special_flags=pygame.BLEND_SOURCE_ALPHA)

but I would need to duplicate a lot of the pygame code for the more complicated group types eg. LayeredUpdates.

Answer 1

The requested feature is implemented with Pygame version 2.3.0. See pygame.sprite.Group.draw. With this version, the code works and you can set the blending mode directly

group.draw(surface, special_flags=pygame.BLEND_SOURCE_ALPHA)

---

Before Pygame 2.3.0, this optional parameter did not exist. If you are using an older version of Pygame, you must implement a workaround.

Draw the sprite with a "for" loop. I suggest to write a function for this or implement your own MyGroup class derived from pygame.sprite.Group:

class MyGroup(pygame.sprite.Group):
    def __init__(self, *args):
        super().__init__(*args) 
    def draw(self, surface, special_flags=0):
        for sprite in self:
            surface.blit(sprite.image, sprite.rect, special_flags = special_flags)

group = MyGroup()

group.draw(surface, special_flags=pygame.BLEND_RGBA_MAX)

If you want to do the same for pygame.sprite.LayeredUpdates or pygame.sprite.RenderUpdates, you have to implement all the drawing features of these objects yourself.

---

A completely different solution is to use pygame.sprite.DirtySprite. This class is derived from pygame.sprite.Sprite and has an additional blendmode attribute which is its the special_flags argument of blit.

class MySprite(pygame.sprite.DirtySprite):
    def __init__(self):
        super().__init__() 
        self.dirty = 2
        self.blendmode = pygame.BLEND_RGBA_MIN
        # [...]

Note, DirtySprite only works with pygame.sprite.LayeredDirty and all sprites in the group must be of this type.