You got it backwards. OOP is meant to make things more reusable. It's all about generalization to specification. Example, say you have a vehicle. This is a very general term. And thus forms the heart of our top-level parent class. A vehicle has a few common properties. Weight, type, color, and occupancy (there's more, but we'll just go with these for this example).

So now we get more specific. What kind of vehicle? automobile. So we create an automobile. An automobile has all the same properties of a vehicle, so we extend the vehicle class, inheriting not only the state of vehicle, but its methods as well. So that's how you reuse methods from another class.

If the class is completely different from the other, and needs a method from another, then you could write a helper function in the outer scope and import it where it is needed to keep things rather DRY. You could also just C&P, that's not that hard.