Lazy evaluation of Python class attributes

The so-called delayed calculation of class attributes is to define the attributes of the class as a property, which will only be calculated when accessed, and once accessed, the result will be cached and does not need to be calculated every time.

Advantages

The main purpose of constructing a lazy calculation property is to improve performance

Implementation

class LazyProperty(object):
    def __init__(self, func):
        self.func = func    def __get__(self, instance, owner):
        if instance is None:            return self        else:
            value = self.func(instance)
            setattr(instance, self.func.__name__, value)            return valueimport mathclass Circle(object):
    def __init__(self, radius):
        self.radius = radius    @LazyProperty
    def area(self):
        print 'Computing area'
        return math.pi * self.radius ** 2    @LazyProperty
    def perimeter(self):
        print 'Computing perimeter'
        return 2 * math.pi * self.radius

Description

defines a lazy calculation decorator class LazyProperty. Circle is a class used for testing. The Circle class has three attributes: radius, area, and perimeter. The properties of area and perimeter are decorated by LazyProperty. Let's try the magic of LazyProperty:

>>> c = Circle(2)
>>> print c.area
Computing area
12.5663706144
>>> print c.area
12.5663706144

"Computing area" will be printed every time it is calculated in area(), and after calling c.area twice in a row, "Computing area" will be printed. " was printed only once. This is due to LazyProperty, as long as it is called once, it will not be counted again no matter how many subsequent calls are made.