gj7 对象引用、可变性和垃圾回收

7.1 python变量到底是什么

#python和java中的变量本质不一样，python的变量实质上是一个指针 int str， 便利贴

a = 1
a = "abc"
#1. a贴在1上面
#2. 先生成对象 然后贴便利贴

a = [1,2,3]
b = a
print (id(a), id(b))
print (a is b)
b.append(4)
print (a)

---
1642030876232 1642030876232
True
[1, 2, 3, 4]

7.2 ==和is的区别

is 判断是否是同一个对象，id是否相等

a = [1,2,3,4]
b = [1,2,3,4]

print(a == b)  # 判断值是否相等
print (id(a), id(b))
print (a is b)

c=1
d=1
print(c is d)  # 将一定范围的小整数，内存地址进行了复用

# ---
True
1642030933512 1642030785544
False
True

---

class People:
    pass

person = People()
if type(person) is People:
    print ("yes")

type(person)

---
yes
__main__.People

7.3 del语句和垃圾回收

# cpython中垃圾回收的算法是采用 引用计数
# 当计数器减为0 就会被回收
a = object()
b = a
del a
print(b)
print(a)


class A:
    def __del__(self):   # 当被回收的时候，执行下面的逻辑
        pass

  

7.4 一个经典的错误

为了数据的安全性，使用tuple而不是list类型来存储

def add(a, b):
    a += b
    return a
a = 1
b = 2
c = add(a,b)
print(c)
print(a,b)
# ---
3
1 2
# ---传递进来的list受到了影响
a = [1,2]
b = [3,4]
c = add(a,b)
print(c)
print(a,b)
# ---
[1, 2, 3, 4]
[1, 2, 3, 4] [3, 4]
# ---
a = (1,2)
b = (3,4)
c = add(a,b)
print(c)
print(a,b)

---
(1, 2, 3, 4)
(1, 2) (3, 4)

---

class Company:
    def __init__(self, name, staffs=[]):
        self.name = name
        self.staffs = staffs
    def add(self, staff_name):
        self.staffs.append(staff_name)
    def remove(self, staff_name):
        self.staffs.remove(staff_name)

com1 = Company("com1", ["lewen1", "lewen2"])
com1.add("lewen3")
com1.remove("lewen1")
print("com1.staffs:",com1.staffs)

com2 = Company("com2") # 使用了默认的[],list又是可变的对象
com2.add("lewen")
print(com2.staffs)

print (Company.__init__.__defaults__) # 没有传递参数时，都使用默认的【】值

com3 = Company("com3")   # 使用了默认的[],list又是可变的对象
com3.add("lewen5")
print (com2.staffs)
print (com3.staffs)
print (com2.staffs is com3.staffs)
# ---
com1.staffs: ['lewen2', 'lewen3']
['lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5', 'lewen']
(['lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5', 'lewen'],)
['lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5']
['lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5', 'lewen', 'lewen5']
True

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