Python: re-defining a class method at runtime

I've defined a class like this:

class MyClass:
  def GetValue(self):
    return 5
  def PrintValue(self):
    print self.GetValue()

For some instances of MyClass I need to re-define the GetValue() dinamically, i.e. something like this:

def GetAGoodValue(self):
  return 7

oneObject=MyClass()
oneObject.GetValue=GetAGoodValue
oneObject.PrintValue()

After re-defining I get the errror:

TypeError: not enough arguments; expected 1, got 0

If inside the PrintValue method I code instead:

print self.GetValue(self)

then the above code works, but only for those MyClass instances where the GetValue method is re-defined. Instances where the GetValue method is not re-defined yeld the error:

TypeError: too many arguments; expected 1, got 2

Any suggestion?