Create typing-final.md

Author: williln

python/typing-final.md

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+# `typing.final` 
+
+## Links 
+
+- [Python docs: `typing.final`](https://docs.python.org/3/library/typing.html#typing.final)
+
+Adding the `@final` decorator to a class or class method indicates that the object cannot be overridden. You can't subclass that class and you can't override that method in a subclass. 
+
+You can add it to either a class or a method on a class. 
+
+Example from the docs: 
+
+```python
+class Base:
+    @final
+    def done(self) -> None:
+        ...
+class Sub(Base):
+    def done(self) -> None:  # Error reported by type checker
+        ...
+
+@final
+class Leaf:
+    ...
+class Other(Leaf):  # Error reported by type checker
+    ...
+```