Python3入门到精通——一切皆对象与魔法函数

作者： Daniel Meng

GitHub： LibertyDream

博客：明月轩

本系列教程采用知识共享署名-非商业性使用-相同方式共享 2.5 中国大陆许可协议

Java 有类（class）和对象（object）的分别，函数、方法更是独一档。但在 Python 中，对不起，即使是包、模块，它们都是对象，Python 比 Java 在对象化上更彻底，这是其一。

其二，既然都是对象，那么函数、类就可以当作对象使用，主要有四点：

1. 能赋值给变量

def obj_func(name):
    print('call me %s' % name)

var_obj = obj_func
var_obj('Daniel Meng')

call me Daniel Meng

class Obj_Class(object):

    def __init__(self, name):
        print('I am %s' % name)

class_var_obj = Obj_Class
class_var_obj('Daniel Meng')

I am Daniel Meng

<__main__.Obj_Class at 0x1967c314550>

1. 可以添加到集合对象中

obj_lst = ['a', class_var_obj, var_obj]
obj_lst

['a', __main__.Obj_Class, <function __main__.obj_func(name)>]

1. 可以当作参数传给函数

def another_call_me(some_func, name):
    some_func(name)

another_call_me(obj_func, 'Daniel Meng')

call me Daniel Meng

1. 可以作为函数返回值

def out_func(name):
    local_name = name
    def inner_func():
        print("Don't call me %s" % local_name)
    return inner_func

var_obj = out_func('Daniel Meng')
var_obj()

Don't call me Daniel Meng

type、object 和 class 的关系

type生成class，包括基础的str,list以及我们创建的类，所有类都有基类，顶级基类object是例外，其基类为空，type继承自object，object是type的一个实例，type作为一个对象，是自己创建了自己（指向自身的指针）

str_obj = 'abc'
type(str_obj)

str

type(str)

type

class MyClass:
    pass

class_obj = MyClass()
type(class_obj)

__main__.MyClass

type(MyClass)

type

type(type) # 指向自己的指针，自己创建自己

type

print('str 父类：%s' % str.__base__)
print('MyClass 父类：%s' % MyClass.__base__)
print('type 父类：%s' % type.__base__)

str 父类：<class 'object'>
MyClass 父类：<class 'object'>
type 父类：<class 'object'>

回头看常见内置类型

Python 中一切皆对象，这里回顾一下对象的三个基本特征——类型（type），身份（id）和值

name = 'Daniel Meng' # 赋值

type(name) # 类型是字符串

str

id(name) # 指向内存位置的编号

1745840344304

从对象角度，会发现 Python 中的内置类型比已知和想象中的要丰富的多。

• None（全局唯一）
• 数值类型
  • int
  • float
  • complex
  • bool
• 迭代类型
• 序列类型（可迭代）
  • list
  • array
  • range
  • tuple
  • str
  • bytes,memoryview
• 映射（dict)
• 集合
  • set
  • frozenset
• 上下文管理类型（with）
• 其他
  • 模块
  • class 和实例
  • 函数
  • 方法
  • 代码（没错，代码也会变成对象>_<）
  • object
  • type
  • ellipsis
  • notimplemented

魔法函数

Python 中的魔法函数格式为__FunctionName__，前后双下滑线，中间函数名。魔法函数是 Python 内部定义好的，用于增强函数、类的功能且独立于二者

class Class(object):

    def __init__(self, student_lst):  # 魔法函数，初始化对象
        self.student_lst = student_lst

    def __getitem__(self,item):  # 魔法函数，尝试一次访问 0，1，2…… 号成员
        return self.student_lst[item]

demo_class = Class(['Tom','Jerry','Mark'])

for stu in demo_class:
    print(stu)

Tom
Jerry
Mark

正是通过各种各样的魔法函数以及魔法函数的组合、嵌套，Python 才能灵活构建出行为表现各异的数据模型。相应的，如果想要对象具有某种特性，比如可迭代，可获取长度，都要事先实现某些魔法函数才行

Python 中的魔法函数大体归类如下

• 数学运算型
  • 一元运算
  • 二元运算
  • 算术运算
  • 反向运算
  • 增量赋值运算
  • 位运算
  • 反向位运算
  • 增量位运算
• 非数学运算型
  • 字符串表示
  • 集合、序列相关
  • 迭代相关
  • 可调用
  • with 上下文管理器
  • 数值转换
  • 元类相关
  • 属性相关
  • 属性描述符
  • 协程

具体内容涉及到相关章节时会细谈，下面列出了部分重要的魔法函数及相关说明

Initialization and Construction	Description
__new__(cls, other)	To get called in an object's instantiation.
__init__(self, other)	To get called by the __new__ method.
__del__(self)	Destructor method.

Unary operators and functions	Description
__pos__(self)	To get called for unary positive e.g. +someobject.
__neg__(self)	To get called for unary negative e.g. -someobject.
__abs__(self)	To get called by built-in abs() function.
__invert__(self)	To get called for inversion using the ~ operator.
__round__(self,n)	To get called by built-in round() function.
__floor__(self)	To get called by built-in math.floor() function.
__ceil__(self)	To get called by built-in math.ceil() function.
__trunc__(self)	To get called by built-in math.trunc() function.

Augmented Assignment	Description
__iadd__(self, other)	To get called on addition with assignment e.g. a +=b.
__isub__(self, other)	To get called on subtraction with assignment e.g. a -=b.
__imul__(self, other)	To get called on multiplication with assignment e.g. a *=b.
__ifloordiv__(self, other)	To get called on integer division with assignment e.g. a //=b.
__idiv__(self, other)	To get called on division with assignment e.g. a /=b.
__itruediv__(self, other)	To get called on true division with assignment
__imod__(self, other)	To get called on modulo with assignment e.g. a%=b.
__ipow__(self, other)	To get called on exponentswith assignment e.g. a **=b.
__ilshift__(self, other)	To get called on left bitwise shift with assignment e.g. a<<=b.
__irshift__(self, other)	To get called on right bitwise shift with assignment e.g. a >>=b.
__iand__(self, other)	To get called on bitwise AND with assignment e.g. a&=b.
__ior__(self, other)	To get called on bitwise OR with assignment e.g. a|=b.
__ixor__(self, other)	To get called on bitwise XOR with assignment e.g. a ^=b.

Type Conversion Magic Methods	Description
__int__(self)	To get called by built-int int() method to convert a type to an int.
__float__(self)	To get called by built-int float() method to convert a type to float.
__complex__(self)	To get called by built-int complex() method to convert a type to complex.
__oct__(self)	To get called by built-int oct() method to convert a type to octal.
__hex__(self)	To get called by built-int hex() method to convert a type to hexadecimal.
__index__(self)	To get called on type conversion to an int when the object is used in a slice expression.
__trunc__(self)	To get called from math.trunc() method.

String Magic Methods	Description
__str__(self)	To get called by built-int str() method to return a string representation of a type.
__repr__(self)	To get called by built-int repr() method to return a machine readable representation of a type.
__unicode__(self)	To get called by built-int unicode() method to return an unicode string of a type.
__format__(self, formatstr)	To get called by built-int string.format() method to return a new style of string.
__hash__(self)	To get called by built-int hash() method to return an integer.
__nonzero__(self)	To get called by built-int bool() method to return True or False.
__dir__(self)	To get called by built-int dir() method to return a list of attributes of a class.
__sizeof__(self)	To get called by built-int sys.getsizeof() method to return the size of an object.

Attribute Magic Methods	Description
__getattr__(self, name)	Is called when the accessing attribute of a class that does not exist.
__setattr__(self, name, value)	Is called when assigning a value to the attribute of a class.
__delattr__(self, name)	Is called when deleting an attribute of a class.

Operator Magic Methods	Description
__add__(self, other)	To get called on add operation using + operator
__sub__(self, other)	To get called on subtraction operation using - operator.
__mul__(self, other)	To get called on multiplication operation using * operator.
__floordiv__(self, other)	To get called on floor division operation using // operator.
__div__(self, other)	To get called on division operation using / operator.
__mod__(self, other)	To get called on modulo operation using % operator.
__pow__(self, other[, modulo])	To get called on calculating the power using ** operator.
__lt__(self, other)	To get called on comparison using < operator.
__le__(self, other)	To get called on comparison using <= operator.
__eq__(self, other)	To get called on comparison using == operator.
__ne__(self, other)	To get called on comparison using != operator.
__ge__(self, other)	To get called on comparison using >= operator.

理解魔法函数，对于写出高性能 Python 程序有巨大帮助。比如说对字符串的两个魔法函数__str__，__repr__。前者是对象字符串格式化时调用的，后者是开发环境下调用的

class StrClass(object):
    def __init__(self, strings):
        self.str = strings

    def __str__(self):
        return ','.join(self.str)

    def __repr__(self):
        return '_'.join(self.str)

sample_str = StrClass(['am','is','are'])

sample_str  # 调用 obj.__repr__()，一般开发环境下终端上不会有输出结果

am_is_are

print(sample_str)  # 调用 obj.__str__()

am,is,are

再比如__len__，当我们使用len()方法时，会自动寻找对象下的__len__。但对于原生内置类型 list、set 等，因为是用 C 实现的，运行时 Cpython 解释器会取巧直接访问 C 程序中内部维护的变量值，从而实现高效访问