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1. num,list ,tuple

结论：num ,tuple 作为参数，自身不会因为函数的原因而改变，list 为可变量，会因为函数变而变。

• 测试

def f(a, b):a += breturn aif __name__ == "__main__":x = 1y = 2result = f(x, y)print("*"*40)print("num 数字作为参数")print(f"x= {x}")print(f"y= {y}")print(f"result={result}")print("*"*40,'\n')x1 = [1,2]y1 = [3,4]result1 = f(x1, y1)print("*"*40)print("list 数字作为参数")print(f"x1= {x1}")print(f"y1= {y1}")print(f"result1={result1}")print("*"*40,'\n')x2 = (1,2)y2 = (3,4)result2 = f(x2, y2)print("*"*40)print("tuple 数字作为参数")print(f"x2= {x2}")print(f"y2= {y2}")print(f"result2={result2}")print("*"*40)

• 结论：

****************************************
num 数字作为参数
x= 1
y= 2
result=3
**************************************** ****************************************
list 数字作为参数
x1= [1, 2, 3, 4]
y1= [3, 4]
result1=[1, 2, 3, 4]
**************************************** ****************************************
tuple 数字作为参数
x2= (1, 2)
y2= (3, 4)
result2=(1, 2, 3, 4)

2. list 作为默认值导致共享同一列表

没有指定初始乘客的HauntedBus实例会共享同一乘客列表

class HauntedBus:"""备受幽灵乘客折磨的校车"""# 默认值为list ，会导致新实例化的bus2 & bus3共享同一列表def __init__(self, passengers=[]):# 设置可变类型作为参数默认值self.passengers = passengersdef pick(self, name):self.passengers.append(name)def drop(self, name):try:self.passengers.remove(name)except ValueError as e:print("error for ", e)if __name__ == "__main__":bus1 = HauntedBus(['Alice', 'Bill'])print(f"bus1.passengers={bus1.passengers}")bus1.pick('Charlie')bus1.drop('Alice')print(f"bus1.passengers={bus1.passengers}")bus2 = HauntedBus()bus2.pick('Carrie')print(f"bus2.passengers={bus2.passengers}")bus3 = HauntedBus()print(f"bus3.passengers={bus3.passengers}")

• 结果

bus1.passengers=['Alice', 'Bill']
bus1.passengers=['Bill', 'Charlie']
bus2.passengers=['Carrie']  # bus2 和bus3 没有默认值的情况下共享同一列表
bus3.passengers=['Carrie']

3. 防御可变参数

• 缺点： 用list 作为形参，会改变实参的值

class TwilightBus:"""让乘客销声匿迹的校车"""def __init__(self, passengers=None):if passengers is None:self.passengers = []else:# 用 passengers 作为参数传入，会导致self.passengers，#                                  passengers ,basketball_team,# 都为列表['sue', 'tina', 'maya', 'diana', 'pat'] 的别名，所以任一改变，# 均可改变['sue', 'tina', 'maya', 'diana', 'pat']的值self.passengers = passengersdef pick(self, name):self.passengers.append(name)def drop(self, name):try:self.passengers.remove(name)except ValueError as e:print("Error for ", e)if __name__ == "__main__":basketball_team = ['sue', 'tina', 'maya', 'diana', 'pat']bus = TwilightBus(basketball_team)print(f"before : basketball_team={basketball_team}")bus.drop('tina')bus.drop('jason')bus.drop('pat')print(f"after : basketball_team={basketball_team}")

• 结果：

before : basketball_team=['sue', 'tina', 'maya', 'diana', 'pat']
Error for  list.remove(x): x not in list
# 列表在经过函数后居然发生变化
after : basketball_team=['sue', 'maya', 'diana']  

• 方案：
  应该把参数值的副本赋值给self.passengers,
  错误：self.passengers = passengers
  正确：self.passengers = list(passengers)

class TwilightBus:"""让乘客销声匿迹的校车"""def __init__(self, passengers=None):if passengers is None:self.passengers = []else:# 用 passengers 作为参数传入，会导致self.passengers，passengers ,basketball_team,# 都为列表['sue', 'tina', 'maya', 'diana', 'pat'] 的别名，所以任一改变，均可改变['sue', 'tina', 'maya', 'diana', 'pat']的值self.passengers = passengersdef pick(self, name):self.passengers.append(name)def drop(self, name):try:self.passengers.remove(name)except ValueError as e:print("Error for ", e)class OkBus:def __init__(self, passengers=None):if passengers is None:self.passengers = []else:# 创建 passengers 的副本，并且赋值给 self.passengers# 重点 list(passengers)!!!!!!!self.passengers = list(passengers)def pick(self, name):self.passengers.append(name)def drop(self, name):try:self.passengers.remove(name)except ValueError as e:print("Error for ", e)if __name__ == "__main__":basketball_team = ['sue', 'tina', 'maya', 'diana', 'pat']bus = TwilightBus(basketball_team)print("*"*40)print(f"before : basketball_team={basketball_team}")bus.drop('tina')bus.drop('jason')bus.drop('pat')print(f"after : basketball_team={basketball_team}")print("*"*40)basketball_teamok = ['sue', 'tina', 'maya', 'diana', 'pat']busok = OkBus(basketball_teamok)print(f"before : basketball_teamok={basketball_teamok}")busok.drop('tina')busok.drop('pat')print(f"after : basketball_teamok={basketball_teamok}")print("*"*40)

• 结果

****************************************
# self.passengers = passengers 的结果，会影响原来的参数列表 basketball_team
before : basketball_team=['sue', 'tina', 'maya', 'diana', 'pat']
Error for  list.remove(x): x not in list
after : basketball_team=['sue', 'maya', 'diana']
****************************************
# self.passengers = list(passengers) 的结果，不影响 原来的参数列表 basketball_team
before : basketball_teamok=['sue', 'tina', 'maya', 'diana', 'pat']
after : basketball_teamok=['sue', 'tina', 'maya', 'diana', 'pat']
****************************************

4. 结论

当函数用参数时，尽量用不可变作为形参，比如数字，元组，如果用可变类型作为变量，会导致传入的值变化，为了解决这个问题，判断参数传入的是不是None,并且重新创建新的list

class Bus:def __init__(self,passengers=None):if passengers is None:self.passengers = []else:self.passengers = list(passengers)