Autosave: 2023-02-17 12:05:32

Programming_Languages/Python/Syntax/Class_inheritance_in_Python.md

@@ -0,0 +1,68 @@
+---
+categories:
+  - Programming Languages
+tags: [python, OOP]
+---
+
+# Class inheritance in Python
+
+We distinguish the **parent/source/superclass** class and the **subclass** that **extends** it.
+
+A subclass can have more than one parent class that it extends, but this is atypical.
+
+> The subclass will inherit all attributes and methods of the superclass unless it overrides them.
+
+In addition to overriding the methods of the parent we can extend them in the child. This is a bit like using an abstract.
+
+## Example of class inheritance
+
+```py
+
+class Person:
+    """ Our superclass
+    """
+    def __init__(self, name, age):
+        self.name = name
+        self.age = age
+
+    def birthday(self):
+        print('Happy birthday you were', self.age)
+        self.age += 1
+        print('You are now', self.age)
+
+    def __str__(self):
+        return self.name + ' is ' + str(self.age)
+
+
+class Employee(Person):
+    """ Our subclass that extends `Person`
+    """
+    def __init__(self, name, age, id):
+        super().__init__(name, age)
+        self.id = id
+
+    def calculate_pay(self, hours_worked):
+        rate_of_pay = 7.50
+        if self.age >= 21:
+            rate_of_pay += 2.50
+        return hours_worked * rate_of_pay
+
+    def __str__(self):
+        return super().__str__() + ' - id(' + str(self.id) + ')'
+```
+
+### Key points
+
+- We pass the name of the parent class to the child class as a parameter
+- We must include all of the parent class's attributes in the constructor of the subclass
+- We must use a special `super().__init__(..,...)` method to enact the extension of the parent. This method references the `__init__` class in the parent.
+
+## Protected/private methods and attributes
+
+> Protected methods and attributes are private to the class but can be accessed from any subclass
+
+> Private methods cannot be accessed from anywhere outside of the class
+
+We declare private methods and attributes with a double underscore (`__`). We declare protected methods and attributes with a single underscore (`_`).
+
+This is **convention**, there are not actually private and protected attributes or properties.

Programming_Languages/Python/Syntax/Classes_in_Python.md

@@ -27,24 +27,112 @@ class Person:
         self.age += 1
         print('You are now', self.age)
 
-
 p1 = Person('John', 36)
 p2 = Person('Thomas', 34)
 print(p1)
 print(p2)
 
 print(p1.name)
-# <__main__.Person object at 0x102e75510>
-# <__main__.Person object at 0x102e75511>
+
 # John
 ```
 
 Key points to note:
 
 - The `__init__` method is the constructor function and must exist on every class to define the properties of the class
-- Every object that the class instantiates is a unique intem in memory as indicated by the hexadecimal log.
 - `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 JS)
 - As with functions, we can use docstrings to document the class. What you write here will show up in Intellisense etc
 
-## The `str` method
+## More complex class
+
+```py
+class Person:
+	""" An example class to hold a persons name and age"""
+
+	def __init__(self, name, age):
+		self.name = name
+		self.age = age
+
+	def __str__(self):
+		return self.name + ' is ' + str(self.age)
+
+	def birthday(self):
+		print ('Happy birthday you were', self.age)
+		self.age += 1
+		print('You are now', self.age)
+
+	def calculate_pay(self, hours_worked):
+		rate_of_pay = 7.50
+		if self.age >= 21:
+			rate_of_pay += 2.50
+		return hours_worked * rate_of_pay
+
+	def is_teenager(self):
+		return self.age < 20
+```
+
+## Object references
+
+When you log a class you get a reference to its hexadecimal [memory](/Computer_Architecture/Memory/Memory.md) reference.
+
+```py
+p1 = Person('John', 36)
+p2 = Person('Thomas', 34)
+
+print(p1)
+print(p2)
+
+# <__main__.Person object at 0x102e75510>
+# <__main__.Person object at 0x102e75511>
+```
+
+This shows each object is unique. You can also generate a specific ID with the `id()` method:
+
+```py
+print(id(p1))
+print(id(p2))
+
+# 4379088272
+# 4379088656
+```
+
+## Copying objects
+
+The same principle that applies to [copying functions](/Programming_Languages/Python/Syntax/Functions_in_Python.md) applies to copying objects created through classes: redeclaration results in a duplicate entity. Thus changes to the duplicate will affect the original.
+
+This becomes obvious when you use `id()` but otherwise might not be apparent. To copy you should therefore declare a new instance of the class.
+
+### The `str` method
+
+The memory reference isn't very helpful for viewing the data contents. To get around this we can use the builtin `__string__` method which automatically logs whatever properties you put in there:
+
+```py
+class Person:
+    """ An example class to hold a persons name and age"""
+
+    def __init__(self, name, age):
+        self.name = name
+        self.age = age
+
+    def __str__(self):
+        return self.name + ' is ' + str(self.age)
+
+p3 = Person('Thomas', 34)
+print(p3)
+# Thomas is 34
+```
+
+## Deleting objects
+
+You might want to delete an object reference because:
+
+- the variable referencing the object goes out of scope
+- the variable is set to `None`
+
+After the `del` statement is applied to a variable that holds an object, the object will no longer be available and any attempt to reference it will result in an error.
+
+```py
+p1 = Person('J-Man', 76)
+del p1
+```

_meta/Topic_Log.md

@@ -19,6 +19,7 @@
   - Modules
   - Error handling
   - Testing
+  - I/O
 
 ## Bash
 
@@ -59,6 +60,8 @@
 ## NodeJS
 
 - Build examples of read stream and write stream
+- Can you override parent class methods in JS subclass?
+  - If not in JS, can you do this in TS?
 
 ## Git