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 calculated property is to improve performance

Achieved

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 value


import math


class 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

Instructions

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 once for each calculation in area(), but "Computing area" will only be printed once after calling c.area twice. 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.

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