简单赋值运算符在Python中变得复杂

我在python中声明了四个变量[a=1，b=2，c=3，d=0]，并使用“，”和“=”（简单赋值运算符）在一行代码中交换它们

我得到了很多答案，并且感到困惑。请帮帮我

案例1:

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例1的输出：

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例2:

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例2的输出：

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例3:

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例3的输出：

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例4:

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

案例4的输出：

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

困惑是：

a=1
b=2
c=3
d=0
a=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
b=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = 0

a=1
b=2
c=3
d=0
c=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = (2,3)
d = 0

a=1
b=2
c=3
d=0
d=a,b=b,c
print "a = " + str(a)
print "b = " + str(b)
print "c = " + str(c)
print "d = " + str(d)

a = 2
b = 3
c = 3
d = (2,3)

在3号和4号案例中，输出是正确的（正如我预期的那样）。但在数字1和2的情况下，a的值是2，b的值是3。我希望这个值应该是（2,3）。那么我的代码有什么问题

---

[我的Python版本是2.7]

tl；dr：多个赋值（一行上有多个

=

语句）是从左到右求值，而不是从右到左求值（在求值右侧表达式之后）

使事情复杂化的是，您正在令人兴奋的混合中使用元组赋值和“正常”赋值

元组赋值使用一个赋值运算符，因此要交换两个变量，请使用：

a, b = b, a

右侧必须计算为与左侧变量数量相同的元素元组。你这么做，没关系

现在，在您的示例中，您不仅仅是在解包元组。当左侧仅包含一个变量时，元组不会解包，只是简单地赋值：

a, b = 1, 2
a = b, a

变成

（2，1）

当你在同一行上使用多个作业时，乐趣就开始了。这些是从左到右处理的

下面是一个简单的例子：

a = b = c = 1

表示

a

变为

1

，然后

b

，然后

c

现在我们可以了解每种情况：

a=a，b=b，c

，其中

a=1

，

b=2

，

c=3

这变成：计算

b，c

->

（2，3）

，然后将其分配给

a

->

a=（2，3）

。然后将其分配给

a，b

，因此

a=2

，

b=3

。结果：

a=2

，

b=3

，

c=3

b=a，b=b，c

，其中

a=1

，

b=2

，

c=3

与之前的情况相同，但现在先设置

b=（2，3）

，然后再设置

b=3

，结果与情况1相同

c=a，b=b，c

，其中

a=1

，

b=2

，

c=3

右侧的输入与案例1相同。现在我们首先设置

c=（2,3）

。预期的最终结果，

a=2

，

b=3

，

c=（2,3）

d=a，b=b，c

，其中

a=1

，

b=2

，

c=3

与案例3相同。但是现在我们改为设置

d

。没有意外

让您感到困惑的是，在计算了右侧之后，作业是从左到右处理的，而不是从右到左处理的

对于这样的情况，实际上运行代码（包装在函数中）最简单的方法是通过以下步骤来分解python字节码：

>>> import dis
>>> def f(): a=a,b=b,c
... 
>>> dis.dis(f)
  1           0 LOAD_FAST                0 (b)
              3 LOAD_GLOBAL              0 (c)
              6 BUILD_TUPLE              2
              9 DUP_TOP             
             10 STORE_FAST               1 (a)
             13 UNPACK_SEQUENCE          2
             16 STORE_FAST               1 (a)
             19 STORE_FAST               0 (b)
             22 LOAD_CONST               0 (None)
             25 RETURN_VALUE        

这是第一例,；请注意，在

构建元组

和

DUP\u TOP

操作码（后者在堆栈上创建一个额外的副本以提供额外的分配）之后，首先发生的是

在
a
上执行的
存储快速
操作，然后是
解包序列
（元组分配操作码），然后将结果存储到
a
和
b
中