Python "global" weirdness

I'm sure there's a rational explanation for this behaviour, but at the moment is slightly baffling.

class SomeObj:
 def __init__(self):
  self.prop = 1

class SomeClass:
 global objInst
 def __init__(self):
  self.reference = objInst
  self.reference.prop += 1
  
if __name__ == '__main__':
 global objInst
 objInst = SomeObj()
 clsInst = SomeClass()
 print "original:" + str(objInst.prop)
 print "clsInst:" + str(clsInst.reference.prop)

# output:
# >>> x:2
# >>> y:2

This is all good, and expected: you get a reference to a global object and manipulate it. But what happens if you try to rebind that global name straight in the __init__ method?

[..]
class SomeClass:
 global objInst 
 def __init__(self):  
  self.reference = objInst
  self.reference.prop += 1
  objInst = SomeObj()
[..]  
# output:
Traceback (most recent call last):
  File "test.py", line 16, in 
    clsInst = SomeClass()
  File "test.py", line 9, in __init__
    self.reference = objInst
UnboundLocalError: local variable 'objInst' referenced before assignment

And it gets even more weird! If you move the "global objInst" declaration inside the __init__ method, it works as expected: objInst is bound to a new object with a different state from the one in self.reference. But if you keep the declaration at class level, and simply move the rebinding out in a new method (separated from __init__), you don't get an error but python does not bind the global to a new object.

I guess it's somehow all a matter of context, and it probably makes perfect sense to programming-language scientists; it just doesn't to me :)