super not behaving as I expected

[Antoon Pardon]

I have the following program

class slt:
	__slots__ = ()

	def getslots(self):
		print("### slots =", self.__slots__)
		if self.__slots__ == ():
			return []
		else:
			ls = super().getslots()
			ls.extend(self.__slots__)
			return ls

	def __str__(self):
		ls = []
		attrs = self.getslots()
		for attr in attrs:
			ls.append(str(getattr( self, attr)))
		return '->'.join(ls)


class slt1 (slt):
	__slots__ = 'fld1', 'fld2'

	def __init__(self, vl1, vl2):
		self.fld1 = vl1
		self.fld2 = vl2

class slt2(slt1):
	__slots__ = 'fld3',
			
	def __init__(self, vl1, vl2, vl3):
		self.fld1 = vl1
		self.fld2 = vl2
		self.fld3 = vl3

rc1 = slt1(4, 7)
rc2 = slt2(11, 18, 29)

print(rc1)
print(rc2)

When I call this I would expect to see the following:

### slots = ('fld1', 'fld2')
### slots = ()
4->7
### slots = (fld3,)
### slots = ('fld1', 'fld2')
### slots = ()
11->18->29

What I actually get is:

### slots = ('fld1', 'fld2')
Traceback (most recent call last):
  File "slottest", line 39, in <module>
    print(rc1)
  File "slottest", line 15, in __str__
    attrs = self.getslots()
  File "slottest", line 9, in getslots
    ls = super().getslots()
AttributeError: 'super' object has no attribute 'getslots'

-- 
Antoon Pardon