Python多变量赋值困惑

Question

我第一天使用 Python，并对一个非常短的示例感到困惑。希望有人能解释一下为什么这几个版本之间存在差异。请!

V1:输出为1,1,2,3,5,8

a, b = 0, 1
while b < 10:
    print(b)
    a, b = b, a+b

V2:输出为1,2,4,8

a, b = 0, 1
while b < 10:
    print(b)
    a = b
    b = a+b

Answer 1

在第一个版本中，首先评估右手，因此在添加它时 b 并未增加。

要单步执行第一个版本进行几次迭代:

1.

a = 0
b = 1
a, b = 1, 1  # b is 1, and a is 0

2.

a = 1
b = 1
a, b = 1, 2  # b is 1 and a is 1

3.

a = 1
b = 2
a, b = 2, 3  # b is 2 and a is 1

在第二个版本中，b 在添加之前就已分配，因此第二个版本的运行方式如下:

1.

a = 0
b = 1
a = b  # a is now 1.
b = a + b  # b is now 2, because both a and b are 1.

2.

a = 1
b = 2
a = b  # a is now 2.
b = a + b  # b is now 4, because both a and b are 2.