Inheritance method overwrite in some conditions

Question

I need to overwrite a method in a Python class, however this method needs to keep his behaviour in certain condition. An example of what I need:

class DefaultClass(object):
    def __init__(self):
        self.number = 5

    def do_something(self, x):
        self.number = self.number * x 
        return self.number

class ChildClass(DefaultClass):
     def __init__(self):
         self.flag = False

     def do_something(self, x):
         if self.flag is False:
             # do_something method should works as defined in DefaultClass
         else:
             self.number = self.number + x
             return self.number

An important point here, I dont have access to the implementation of the method that I want to overwrite (life is not so simple).

Thanks in advance!