python byRef //复制

Question

我刚接触Python (而且对编程了解不多)，但我记得我读过一篇文章，说python通常不复制值，所以任何a =b语句都会使b指向a。

a = 1
b = a
a = 2
print(b)

给出结果1。这不应该是2吗？

Answer 1

不，结果应该是1。

将赋值运算符( = )视为引用的赋值。

a = 1 #a references the integer object 1
b = a #b and a reference the same object
a = 2 #a now references a new object (2)
print b # prints 1 because you changed what a references, not b

在处理像lists这样的可变对象而不是像int、float和tuple这样的不可变对象时，整个区别是最重要的。

现在考虑以下代码：

a=[]  #a references a mutable object
b=a   #b references the same mutable object
b.append(1)  #change b a little bit
print a # [1] -- because a and b still reference the same object 
        #        which was changed via b.

Answer 2

当您执行b = a时，它会使b引用与a相同的值。然后，当您执行a = 2时，它会引用一个新值。B不受影响。

Python中的赋值规则：

1. 赋值只是使名称引用某个名称的value.
2. Assignment，而不会影响引用旧值的其他名称。永远不会隐式复制
3. 数据。

Answer 3

@mgilson有一个很棒的answer，但我发现它有点难以理解。我把这个答案放进去，试图用一种不同的方式来解释它。

a = 1 # This makes a point to a location in memory where the Integer(1) is located
b = a # This makes b point to the same place.
a = 2 # This makes a point to a different location in memory where Integer(2) is located
print(b) # b still points to the place where Integer(1) was stored, so it prints out 1.