Merge pull request #582 from realpython/python-property

Sample code for the article on properties

python-property/README.md

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+# Python's property(): Add Managed Attributes to Your Classes
+
+This folder provides the code examples for the Real Python tutorial [Python's property(): Add Managed Attributes to Your Classes](https://realpython.com/python-property/).

python-property/circle_v1.py

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+class Circle:
+    def __init__(self, radius):
+        self._radius = radius
+
+    def _get_radius(self):
+        print("Get radius")
+        return self._radius
+
+    def _set_radius(self, value):
+        print("Set radius")
+        self._radius = value
+
+    def _del_radius(self):
+        print("Delete radius")
+        del self._radius
+
+    radius = property(
+        fget=_get_radius,
+        fset=_set_radius,
+        fdel=_del_radius,
+        doc="The radius property.",
+    )

python-property/circle_v2.py

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+class Circle:
+    def __init__(self, radius):
+        self._radius = radius
+
+    @property
+    def radius(self):
+        """The radius property."""
+        print("Get radius")
+        return self._radius
+
+    @radius.setter
+    def radius(self, value):
+        print("Set radius")
+        self._radius = value
+
+    @radius.deleter
+    def radius(self):
+        print("Delete radius")
+        del self._radius

python-property/circle_v3.py

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+class Circle:
+    def __init__(self, radius):
+        self.radius = radius
+
+    @property
+    def radius(self):
+        return self._radius
+
+    @radius.setter
+    def radius(self, value):
+        self._radius = float(value)
+
+    @property
+    def diameter(self):
+        return self.radius * 2
+
+    @diameter.setter
+    def diameter(self, value):
+        self.radius = value / 2

python-property/circle_v4.py

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+from time import sleep
+
+
+class Circle:
+    def __init__(self, radius):
+        self.radius = radius
+        self._diameter = None
+
+    @property
+    def diameter(self):
+        if self._diameter is None:
+            sleep(0.5)  # Simulate a costly computation
+            self._diameter = self.radius * 2
+        return self._diameter

python-property/circle_v5.py

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+from time import sleep
+
+
+class Circle:
+    def __init__(self, radius):
+        self.radius = radius
+
+    @property
+    def radius(self):
+        return self._radius
+
+    @radius.setter
+    def radius(self, value):
+        self._diameter = None
+        self._radius = value
+
+    @property
+    def diameter(self):
+        if self._diameter is None:
+            sleep(0.5)  # Simulate a costly computation
+            self._diameter = self._radius * 2
+        return self._diameter

python-property/circle_v6.py

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+from functools import cached_property
+from time import sleep
+
+
+class Circle:
+    def __init__(self, radius):
+        self.radius = radius
+
+    @cached_property
+    def diameter(self):
+        sleep(0.5)  # Simulate a costly computation
+        return self.radius * 2

python-property/circle_v7.py

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+from functools import cache
+from time import sleep
+
+
+class Circle:
+    def __init__(self, radius):
+        self.radius = radius
+
+    @property
+    @cache
+    def diameter(self):
+        sleep(0.5)  # Simulate a costly computation
+        return self.radius * 2

python-property/circle_v8.py

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+import logging
+
+logging.basicConfig(
+    format="%(asctime)s: %(message)s",
+    level=logging.INFO,
+    datefmt="%H:%M:%S",
+)
+
+
+class Circle:
+    def __init__(self, radius):
+        self._msg = '"radius" was %s. Current value: %s'
+        self.radius = radius
+
+    @property
+    def radius(self):
+        logging.info(self._msg % ("accessed", str(self._radius)))
+        return self._radius
+
+    @radius.setter
+    def radius(self, value):
+        try:
+            self._radius = float(value)
+            logging.info(self._msg % ("mutated", str(self._radius)))
+        except ValueError:
+            logging.info('validation error while mutating "radius"')

python-property/currency_v1.py

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+class Currency:
+    def __init__(self, units, cents):
+        self.units = units
+        self.cents = cents
+
+    # Currency implementation...

python-property/currency_v2.py

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+CENTS_PER_UNIT = 100
+
+
+class Currency:
+    def __init__(self, units, cents):
+        self._total_cents = units * CENTS_PER_UNIT + cents
+
+    @property
+    def units(self):
+        return self._total_cents // CENTS_PER_UNIT
+
+    @units.setter
+    def units(self, value):
+        self._total_cents = self.cents + value * CENTS_PER_UNIT
+
+    @property
+    def cents(self):
+        return self._total_cents % CENTS_PER_UNIT
+
+    @cents.setter
+    def cents(self, value):
+        self._total_cents = self.units * CENTS_PER_UNIT + value
+
+    # Currency implementation...

python-property/node.py

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+class TreeNode:
+    def __init__(self, data):
+        self._data = data
+        self._children = []
+
+    @property
+    def children(self):
+        return self._children
+
+    @children.setter
+    def children(self, value):
+        if isinstance(value, list):
+            self._children = value
+        else:
+            del self.children
+            self._children.append(value)
+
+    @children.deleter
+    def children(self):
+        self._children.clear()
+
+    def __repr__(self):
+        return f'{self.__class__.__name__}("{self._data}")'

python-property/persons.py

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+class Person:
+    def __init__(self, name):
+        self._name = name
+
+    @property
+    def name(self):
+        return self._name
+
+    @name.setter
+    def name(self, value):
+        self._name = value
+
+    # Person implementation...
+
+
+class Employee(Person):
+    @property
+    def name(self):
+        return super().name.upper()
+
+    # Employee implementation...

python-property/point_v1.py

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+class Point:
+    def __init__(self, x, y):
+        self._x = x
+        self._y = y
+
+    def get_x(self):
+        return self._x
+
+    def set_x(self, value):
+        self._x = value
+
+    def get_y(self):
+        return self._y
+
+    def set_y(self, value):
+        self._y = value

python-property/point_v2.py

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+class Point:
+    def __init__(self, x, y):
+        self._x = x
+        self._y = y
+
+    @property
+    def x(self):
+        return self._x
+
+    @property
+    def y(self):
+        return self._y

python-property/point_v3.py

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+class WriteCoordinateError(Exception):
+    pass
+
+
+class Point:
+    def __init__(self, x, y):
+        self._x = x
+        self._y = y
+
+    @property
+    def x(self):
+        return self._x
+
+    @x.setter
+    def x(self, value):
+        raise WriteCoordinateError("x coordinate is read-only")
+
+    @property
+    def y(self):
+        return self._y
+
+    @y.setter
+    def y(self, value):
+        raise WriteCoordinateError("y coordinate is read-only")

python-property/point_v4.py

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+class Point:
+    def __init__(self, x, y):
+        self.x = x
+        self.y = y
+
+    @property
+    def x(self):
+        return self._x
+
+    @x.setter
+    def x(self, value):
+        try:
+            self._x = float(value)
+            print("Validated!")
+        except ValueError:
+            raise ValueError('"x" must be a number') from None
+
+    @property
+    def y(self):
+        return self._y
+
+    @y.setter
+    def y(self, value):
+        try:
+            self._y = float(value)
+            print("Validated!")
+        except ValueError:
+            raise ValueError('"y" must be a number') from None

python-property/point_v5.py

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+class Coordinate:
+    def __set_name__(self, owner, name):
+        self._name = name
+
+    def __get__(self, instance, owner):
+        return instance.__dict__[self._name]
+
+    def __set__(self, instance, value):
+        try:
+            instance.__dict__[self._name] = float(value)
+            print("Validated!")
+        except ValueError:
+            raise ValueError(f'"{self._name}" must be a number') from None
+
+
+class Point:
+    x = Coordinate()
+    y = Coordinate()
+
+    def __init__(self, x, y):
+        self.x = x
+        self.y = y

python-property/point_v6.py

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+import math
+
+
+class Point:
+    def __init__(self, x, y):
+        self.x = x
+        self.y = y
+
+    @property
+    def distance(self):
+        return round(math.dist((0, 0), (self.x, self.y)))
+
+    @property
+    def angle(self):
+        return round(math.degrees(math.atan(self.y / self.x)), 1)
+
+    def as_cartesian(self):
+        return self.x, self.y
+
+    def as_polar(self):
+        return self.distance, self.angle

python-property/product.py

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+class Product:
+    def __init__(self, name, price):
+        self._name = name
+        self._price = float(price)
+
+    @property
+    def price(self):
+        return f"${self._price:,.2f}"

python-property/rectangle.py

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+class Rectangle:
+    def __init__(self, width, height):
+        self.width = width
+        self.height = height
+
+    @property
+    def area(self):
+        return self.width * self.height