7.1 python变量到底是什么

#pythonjava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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