function that run once for all objects

[Gribouillis]

You can simply override the __init__method after the first instance's creation

class Spam:
    def __init__(self):
        Spam.thingy = "eggs"
        _other_init(self)
        Spam.__init__ = _other_init
        print("Hey! I'm the first Spam instance'")

def _other_init(self):
    print("Initializing", self)

for i in range(3):
    Spam()

Output:
ens@thom:~$ python tmp/initruc.py
Initializing <__main__.Spam object at 0x7fb34a8b14c0>
Hey! I'm the first Spam instance'
Initializing <__main__.Spam object at 0x7fb34a8b14c0>
Initializing <__main__.Spam object at 0x7fb34a8b14c0>