inheritance in python

zk/Classes_in_Python.md

@@ -55,8 +55,9 @@ print(p1.name)
 
 Key points to note:
 
-- The `__init__` method is the constructor function and must exist on every
-  class to define the properties of the class
+- The `__init__` method is the constructor function. You only need to provide it
+  if you are defining properties at instantiation. If it's going to be empty,
+  don't bother.
 - `self` is a reference to the class itself and the object it will create, akin
   to `this` in other languages
 - You must pass `self` as a parameter to every method (this is a difference from
@@ -106,6 +107,32 @@ class Person:
 		return self.age < 20
 ```
 
+## Inheritance
+
+Say that `TableService` is a child of `SqliteService`:
+
+```py
+class TableService(SqliteService):
+```
+
+Say that the parent does something in the constructor, that you want to also do
+in the child, or if you want to change the parent constuctor behaviour, you have
+to invoke `super`:
+
+```py
+class SqliteService:
+    def __init__(self, db_connection):
+        self.connection = db_connection
+        self.cursor = db_connection.cursor()
+
+class TableService(SqliteService):
+    def __init__(self, db_connection):
+        super().__init__(db_connection)
+
+```
+
+See more in [Class inheritance in Python](./Class_inheritance_in_Python.md).
+
 ## Object references
 
 When you log a class you get a reference to its hexadecimal memory reference.