First we have to define the States

So to define states follow the below code.

                   
 columns = {
                state: fields.selection([
                 (new,New),                   
                 (assigned,Assigned),
                 (negotiation,Negotiation),
                 (won,Won),
                 (lost,Lost)], Stage, readonly=True),
            } 

After that you can add States Fields (Object).

def mymod_new(self, cr, uid, ids):
     self.write(cr, uid, ids, { 'state' : 'new' })
     return True
def mymod_assigned(self, cr, uid, ids):
     self.write(cr, uid, ids, { 'state' : 'assigned' })
     return True
def mymod_negotiation(self, cr, uid, ids):
     self.write(cr, uid, ids, { 'state' : 'negotiation' })
     return True
def mymod_won(self, cr, uid, ids):
     self.write(cr, uid, ids, { 'state' : 'won' })
     return True
def mymod_lost(self, cr, uid, ids):
     self.write(cr, uid, ids, { 'state' : 'lost' })
     return True