[pygame]how to copy a surface to an other surface with alpha value?

[flyaflya]

I want to join some surfaces to a new big surface with alpha cannel, I want
the new surface has  same pixels(inclue r,g,b and alpha value) as the pixels
on the source surfaces.
my code as follow:

surf = pygame.Surface((200,200))
surf.blit(surf1, (0,0))
surf.blit(surf2, (0,100))
.....

but these codes can't copy alpha value to the new surface.how can I deal
with it?