Add pure Python implementation

Author: gjbex

source-code/cython/Classes/README.md

@@ -6,6 +6,9 @@ Illustration of using Cython extension types (aka cdef classes).
 
 1. `points.pyx`: implementation of a cdef class, and a Python
    child class thereof.
+1. `points_pure.py`: implementation in Cython's pure Python
+   syntax.
+1. `points_python.py`: Python implementation of the class.
 1. `driver.py`: Python script that uses both classes.
 1. `setup.py`: Python installation file to build the Cython
    extension.

source-code/cython/Classes/points_python.py

@@ -0,0 +1,50 @@
+from math import sqrt
+
+class Point:
+    
+    _x: float
+    _y: float
+    
+    def __init__(self, x: float, y: float) -> None:
+        self.x = x
+        self.y = y
+        
+    def distance(self, other) -> float:
+        return sqrt((self.x - other.x)*(self.x - other.x) + (self.y - other.y)*(self.y - other.y))
+    
+    @property
+    def x(self) -> float:
+        return self._x
+
+    @x.setter
+    def x(self, value: float) -> None:
+        self._x = float(value)
+
+    @property
+    def y(self) -> float:
+        return self._y
+
+    @y.setter
+    def y(self, value: float) -> None:
+        self._y = float(value)
+
+    def __str__(self) -> str:
+        return f"Point({self.x}, {self.y})"
+
+
+class ColoredPoint(Point):
+
+    def __init__(self, x, y, color):
+        super().__init__(x, y)
+        self._color = color
+
+    @property
+    def color(self):
+        return self._color
+
+    @color.setter
+    def color(self, value):
+        self._color = str(value)
+
+    def __str__(self):
+        return f"ColoredPoint({self.x}, {self.y}, {self.color})"