unexpected sub result after overloading operator

[jolinchewjb]

I have below code to test overload of the operator, add is tested ok, sub is not ok, after P1-p3, it becomes empty set, but my sub function is correct, anyone knows why? thanks

class Opera(object):
    
   def __init__(self,opera):
          self.opera=[]
          if  type(opera) in (list,tuple):
              self.opera.extend(opera)
          
   def __add__(self, other):
          return Opera(self.opera+other.opera)
   def __sub__(self, other):
          return Opera(p for p in self.opera if p not in other.opera)
   def __str__(self):
          return 'Operator overload Example{}'.format(self.opera)  
  p1=Opera(['red','Green','Orange'])
  p2=Opera(['go','stop'])
  p3=Opera(['Orange'])

  print ('p1:'+str(p1))
  print ('p2:'+str(p2))
  print ('p3:'+str(p3))
  print ('p1+p2:'+str(p1+p2))
  print ('p1-p3:'+str(p1-p3))