第二章 魔法函数

一. 什么是魔法函数

# 打印class里的list
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
company = Company(['Alice', 'Bob', 'Candy'])
for x in company.employee:
  print(x) # 'Alice', 'Bob', 'Candy'
# for x in company:
#   print(x)
# TypeError: 'Company' object is not iterable

# 直接遍历class
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
  # 添加魔法函数：__getitem__
  def __getitem__(self, item):
    return self.employee[item]
company = Company(['Alice', 'Bob', 'Candy'])
for x in company:
  print(x) # 'Alice', 'Bob', 'Candy'
# for x in company[1:]: 切片
#  print(x) # 'Bob', 'Candy'
# print(len(company)) # TypeError: object of type 'Company' has no len()
print(len(company[:])) # 3，Company并未实现len函数所以直接调用len会出错
# 但是company[:]是先进行切片，切片后会变成list类型，因此可以用len函数

# 长度魔法函数
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
  # 添加魔法函数：__getitem__
  def __getitem__(self, item):
    return self.employee[item]
  def __len__(self):
    return len(self.employee)
company = Company(['Alice', 'Bob', 'Candy'])
print(len(company))

二. python的数据模型以及数据模型对python的影响（数据模型也叫魔法函数）

三. 魔法函数一览

image.png

# 打印对象，打印对象时，python会默认调用str()函数
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
company = Company(['Alice', 'Bob', 'Candy'])
print(company) # <__main__.Company object at 0x000001E264E0FE80>
print(str(company)) #  <__main__.Company object at 0x000001E264E0FE80>

# __str__与__repr__的区别
# 使用__str__
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
  def __str__(self):
    return ','.join(self.employee)
company = Company(['Alice', 'Bob', 'Candy'])
print(company) # Alice,Bob,Candy
company # <__main__.Company at 0x1e264ee1cc0>
# 使用__repr__:开发时调用
class Company:
  def __init__(self, employee_list):
    self.employee = employee_list
  def __repr__(self):
    return ','.join(self.employee)
company = Company(['Alice', 'Bob', 'Candy'])
print(company) # Alice,Bob,Candy
company # Alice,Bob,Candy

# 数值魔法函数
class Num:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    # def __abs__(self):
    #     return abs(self.num)

    def __add__(self, other_instance):
        re_vactor = Num(self.x + other_instance.x, self.y + other_instance.y)
        return re_vactor

    def __str__(self):
        return 'x: {x}, y: {y}'.format(x=self.x, y=self.y)


num1 = Num(2, 3)
num2 = Num(1, 4)
print(num1 + num2) # x: 3, y: 7

在cpython中，内置类型如：list, dict, set 都是用C语言实现的，所以在用len方法时会更快，而自己写的类实现的魔法函数__len__，其实是对对象进行了遍历，没有原生的内置类型快，因此要尽量用python的内置类型