python 一些易错点整理

这篇文章是抄抄写写得来的，纯粹是这个编辑器比笔记的好太多，才在这儿写。

函数参数传递

Python的函数参数传递

对于变量（与对象相对的概念），其实，python函数参数传递可以理解为就是变量传值操作，用C++的方式理解，就是对void*赋值。如果这个变量的值不变，我们看似就是引用，如果这个变量的值改变，我们看着像是在赋值。

自己的理解：传递的值都会复制一份，如果是可变值，函数体内变量值变动时，指针指向的值会改，则看起来像是引用；如果是不可变值，函数体内变量值变动时，会重新赋值，则看起来像赋值。

global 与 nonlocal 比较

python中global与nonlocal比较

nonlocal only works in py3

global关键字用来在函数或其他局部作用域中使用全局变量。如果不修改全局变量，也可以不使用global关键字

nonlocal关键字用来在函数或其他作用域中使用外层（非全局）变量。
亲自动手试后，发现使用了 nonlocal 只会读闭包内的变量，可以隔着多层

init new

Use new when you need to control the creation of a new instance. Use init when you need to control initialization of a new instance.

new is the first step of instance creation. It's called first, and is responsible for returning a new instance of your class. In contrast, init doesn't return anything; it's only responsible for initializing the instance after it's been created. [3]

sf 上一哥们类比： new 看作为 alloc 步骤

A metaclass is just the class of a class. a metaclass's call method controls what happens when call a class. allows you to redefine the instance-creation mechanism from start to finish

class Singleton(type):
    def __init__(self, *args, **kwargs):
        super(Singleton, self).__init__(*args, **kwargs)
        self.__instance = None
    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super(Singleton, self).__call__(*args, **kwargs)
        return self.__instance

__metaclass__ = Singleton

singleton [3]

def singleton(cls):
    cls.__new_original__ = cls.__new__
    
    @functools.wraps(cls.__new__)
    def singleton_new(cls, *args, **kw):
        it = cls.__dict__.get('__it__')
        if it is not None:
            return it
        
        cls.__it__ = it = cls.__new_original__(cls, *args, **kw)
        it.__init_original__(*args, **kw)
        return it
    
    cls.__new__ = singleton_new
    cls.__init_original__ = cls.__init__
    cls.__init__ = object.__init__
    
    return cls