示例中的行为如下

In [1]: a = 1                                                                                                                                                                     

In [2]: b = 2                                                                                                                                                                     

In [3]: a = b                                                                                                                                                                     

In [4]: b+=1                                                                                                                                                                      

In [5]: b                                                                                                                                                                         
Out[5]: 3

In [6]: a                                                                                                                                                                         
Out[6]: 2

在这个例子中,当你 a=b b += 1 ,将1添加到b的操作,创建一个新的整数值 3 因为b和b指向那个值,但a仍然指向旧值 2

In [1]: a = [1]                                                                                                                                                                   

In [2]: b = [2]                                                                                                                                                                   

In [3]: a = b                                                                                                                                                                     

In [4]: b.append(2)                                                                                                                                                               

In [5]: a                                                                                                                                                                         
Out[5]: [2, 2]

In [6]: b                                                                                                                                                                         
Out[6]: [2, 2]

In [7]: b += [3, 4];                                                                                                                                                                        

In [8]: b
Out[8]: [2, 2, 3, 4]

In [9]: a
Out[9]: [2, 2, 3, 4]

现在这里发生了什么?我们改变了 b 但是 a append 或者更新列表 in-place 从那以后 一 乙 结果都更新了!

+= 运算符由定义 __iadd__ int -所有的 __iXXX__ 方法返回int的新实例 list -s公司 __iadd__(self, other) 做 self.extend(other); return self

因此,甚至整数也可以作为一个列表,例如 here

python中的变量是引用。很少有对不可变对象(如字符串、整数等)的引用,而列表和集合是可变的。

Python 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 22:20:52) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
# Consider two variables x & y  with integers 23 and 46. They have immutable references
# Meaning, if y=x, it doesn't mean y will change whenever x is updated.
>>> x=23
>>> print (x)
23
>>> y=46
>>> print (x)
23
>>> print (y)
46
>>> y=x
>>> print (x)
23
>>> print (y)
23
# Let's change the value of x after the assignment
>>> x=99
>>> print (x)
99
# Since it is immutable, the value wont change.
>>> print (y)
23
#Let's consider the mutable reference scenario. a & b are two lists which have mutable references
# Meaning, if b=a, it means b will change whenever a is changed
>>> a = list([11,22,33,87])
>>> print (a)
[11, 22, 33, 87]
>>> b = list([87,53,98,2])
>>> print (b)
[87, 53, 98, 2]
>>> a=b
>>> print (a)
[87, 53, 98, 2]
>>> print (b)
[87, 53, 98, 2]
# Popping the list would alter b
>>> b.pop()
2
>>> print (b)
[87, 53, 98]
# Notice the change in value of a
>>> print (a)
[87, 53, 98]
>>>

他们有这个名字是因为他们“指向”事物。
a b , 乙 指向 2 ,等等。

你所期待的事情会发生:

实际发生了什么: