Python 懒加载属性

Python 懒加载属性

functools.update_wrapper用法参考：https://zhblog.net/go/python/practice/python-functools-updatewrapper

类中__get__特殊方法的使用参考：https://zhblog.net/go/python/practice/python-class-decorator-init-get-call

import functools


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


def lazy_property2(fn):
    attr = '_lazy__' + fn.__name__    

    @property    
    def _lazy_property(self):
        if not hasattr(self, attr):
            setattr(self, attr, fn(self))
        return getattr(self, attr)

    return _lazy_property


class Person(object):
    def __init__(self, name, occupation):
        self.name = name
        self.occupation = occupation
        self.call_count2 = 0    

    @lazy_property    
    def relatives(self):
        # Get all relatives, let's assume that it costs much time.        
        relatives = "Many relatives."        
        return relatives

    @lazy_property2   
    def parents(self):
        self.call_count2 += 1        
        return "Father and mother"

def main():
    """    
    >>> Jhon = Person('Jhon', 'Coder')    
    >>> Jhon.name    
    'Jhon'    
    >>> Jhon.occupation    
    'Coder'    
    # Before we access `relatives`    
    >>> sorted(Jhon.__dict__.items())    
    [('call_count2', 0), ('name', 'Jhon'), ('occupation', 'Coder')]    
    >>> Jhon.relatives    
    'Many relatives.'    
    # After we've accessed `relatives`    
    >>> sorted(Jhon.__dict__.items())    
    [('call_count2', 0), ..., ('relatives', 'Many relatives.')]    
    >>> Jhon.parents    
    'Father and mother'    
    >>> sorted(Jhon.__dict__.items())    
    [('_lazy__parents', 'Father and mother'), ('call_count2', 1), ..., ('relatives', 'Many relatives.')]    
    >>> Jhon.parents    
    'Father and mother'    
    >>> Jhon.call_count2    
    1    
    """