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+In [TypeScript](./Classes_in_TypeScript.md) we can use interfaces to implement
+classes and use `abstract` to create abstract methods:
+
+```ts
+interface Person {
+  firstName: string;
+  calculateAge(): number;
+}
+
+class Programmer implements Person {}
+// Must include the property firstName and the method calculateAge
+```
+
+```ts
+abstract class Person {
+  abstract calculateAge(): number;
+}
+
+class Child extends Person {}
+// Must concretize the abstract calculateAge method from the parent class
+```
+
+The same --or very similar-- functionality can be achieved in Python by
+importing the `abc` module ("Abstract Base Class").
+
+```py
+from abc import ABC, abstractmethod
+
+class Person(ABC):
+    @abstractmethod
+    def calculate_age(self):
+        pass
+
+
+class Child(Person):
+    def calculate_age(self):
+        # Must concretize the abstract method in Person
+```
+
+> Only use `@abstractmethod` if the method _must_ exist on the child and _must_
+> be defined in the child (not the parent). If you just want to redefine a
+> parent method, you can use
+> [normal inheritance](./Class_inheritance_in_Python.md). Python will interpret
+> the methods upwards from the child, before getting to the parent.
diff --git a/zk/Classes.md b/zk/Classes_in_TypeScript.md
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rename to zk/Classes_in_TypeScript.md