延迟加载模式（lazy evaluation）

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延迟加载模式（lazy evaluation）
代码参考引用自：https://github.com/faif/python-patterns/blob/master/lazy_evaluation.py

代码

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
Lazily-evaluated property pattern in Python.
https://en.wikipedia.org/wiki/Lazy_evaluation
References:
bottle
https://github.com/bottlepy/bottle/blob/cafc15419cbb4a6cb748e6ecdccf92893bb25ce5/bottle.py#L270
django
https://github.com/django/django/blob/ffd18732f3ee9e6f0374aff9ccf350d85187fac2/django/utils/functional.py#L19
pip
https://github.com/pypa/pip/blob/cb75cca785629e15efb46c35903827b3eae13481/pip/utils/__init__.py#L821
pyramimd
https://github.com/Pylons/pyramid/blob/7909e9503cdfc6f6e84d2c7ace1d3c03ca1d8b73/pyramid/decorator.py#L4
werkzeug
https://github.com/pallets/werkzeug/blob/5a2bf35441006d832ab1ed5a31963cbc366c99ac/werkzeug/utils.py#L35
"""
class lazy_property(object):
    def __init__(self, function):
        self.function = function
        functools.update_wrapper(self, function)
    def __get__(self, obj, type_):
        if obj is None:
            return self
        val = self.function(obj)
        obj.__dict__[self.function.__name__] = val
        return val
class Person(object):
    def __init__(self, name, occupation):
        self.name = name
        self.occupation = occupation
    @lazy_property
    def relatives(self):
        # Get all relatives, let's assume that it costs much time.
        relatives = "Many relatives."
        return relatives
def main():
    Jhon = Person('Jhon', 'Coder')
    print("Name: {0}    Occupation: {1}".format(Jhon.name, Jhon.occupation))
    print("Before we access `relatives`:")
    print(Jhon.__dict__)
    print("Jhon's relatives: {0}".format(Jhon.relatives))
    print("After we've accessed `relatives`:")
    print(Jhon.__dict__)
### OUTPUT ###
# Name: Jhon    Occupation: Coder
# Before we access `relatives`:
# {'name': 'Jhon', 'occupation': 'Coder'}
# Jhon's relatives: Many relatives.
# After we've accessed `relatives`:
# {'relatives': 'Many relatives.', 'name': 'Jhon', 'occupation': 'Coder'}

分析

装饰器类 lazy_property 中的 __get__()方法，是为该类实现为了描述符（descriptor）。__get__() 方法是在访问类或者类实例的属性时，类实例的 __dict__ 中不存在被访问属性，类的 __dict__ 中也不存在被访问属性的情况下，会调用 __get__() 方法获取被访问属性，即例子代码中的 relatives。在执行语句 print("John's relatives: {0}".format(John.relatives)) 时，实例 Jhon 的 __dict__ 中还没有属性 relatives，所以装饰器类的 __get__ 方法被调用，为实例 Jhon 设置了 revatives 属性，达到延迟加载的效果。

本例说明的情况为，假设类 Person 的实例的 relatives 属性加载特别缓慢，在实例化 Person 类的时候暂时不要初始化该属性，找其他合适的时机再延迟加载。