Describing Properties in Python
- A property is a special attribute that computes its value when accessed
- The following is an example of a property:
>>> class Circle(object):
... def __init__(self, radius):
... self.radius = radius
... @property
... def area(self):
... return math.pi*self.radius**2
... @property
... def perimeter(self):
... return 2*math.pi*self.radius
>>> c = Circle(4)
>>> c.radius
4
>>> c.area
50.26
>>> c.radius = 2
>>> c.area = 2
AttributeError: cant set attribute- In this example,
c.radiusis an instance variable - Then,
c.areaandc.perimeterare properties - This form of property is useful when creating getters
Setting and Deleting Attributes with Properties
- Properties can intercept operations to set and delete an attribute
- This is done by attaching additional setter and deleter methods to a property
- The following contains these methods:
>>> class Foo(object):
... def __init__(self, name):
... self.__name = name
... @property
... def name(self): # getter
... return self.__name
... @name.setter
... def name(self, value): # setter
... if not isinstance(value, str):
... raise TypeError('nope')
... self.__name = value
... @name.deleter
... def name(self): # deleter
... raise TypeError('cannot delete')
>>> f = Foo('Todd')
>>> n = f.name
>>> f.name = 'Monty'
>>> f.name = 45
TypeError: nope
>>> del f.name
TypeError: cannot delete- By doing this, we can implement logic for deleting and setting
- We can also hide attributes with double underscore, while providing access to them via properties
References
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