输出背后的原因是什么？

Question

输入：

x, y = 20, 60
y, x, y = x, y-10, x+10
print(x, y)

输出：

50 30

我期望什么？

• x = 20

• y = 60

• y = x = 20

• x = y-10 = 20-10 = 10

• y = x + 10 = 20

预期输出：

10 20

为什么不是这种情况？是因为首先对表达式求值，然后才给变量赋值？

Answer 1

右侧将进行完全评估，并在分配发生之前变成一个元组。所以你的代码：

x, y = 20, 60
y, x, y = x, y-10, x+10
print(x, y)

可以改写为：
r = (20, 60)
x, y = r # x = 20, y = 60

r = (x, y-10, x+10) # r = (20, 50, 30)
y, x, y = r # This line may lead to confusion
# Tuple unpacking is done left to right so the line above is equivalent to:
# y = 20
# x = 50
# y = 30 --> Note this overwrote the previous y assignment

因此
50 30