Python:fib函数中的for循环不'；我不明白

在Python课程中，我有一个关于斐波那契数的练习。 下面是第4.1.5.7章的完整内容

我的问题是关于函数本身。特别是我对环路内部发生的事情感到困惑：

for i in range(3, n + 1):
    sum = elem1 + elem2
    elem1, elem2 = elem2, sum
return sum

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在第三行中，您可以看到elem1逗号elem2等于elem2逗号和。这是什么意思

有人能一步一步地解释我如何解释这个循环的内容吗

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• 第4.1.5.7章的完整文本：

一些简单函数：斐波那契数 你熟悉斐波那契数吗

它们是使用非常简单的规则构建的整数序列：

序列的第一个元素等于一（Fib1=1） 第二个也等于一（Fib2=1） 后面的每个数都是前面两个数的和（Fibi=Fibi-1+Fibi-2） 以下是一些Fibonacci数：

fib1 = 1
fib2 = 1
fib3 = 1 + 1 = 2
fib4 = 1 + 2 = 3
fib5 = 2 + 3 = 5
fib6 = 3 + 5 = 8
fib7 = 5 + 8 = 13

您认为如何将其作为一个函数来实现

让我们创建fib函数并测试它。这是：

def fib(n):
    if n < 1:
         return None
    if n < 3:
        return 1

    elem1 = elem2 = 1
    sum = 0
    for i in range(3, n + 1):
        sum = elem1 + elem2
        elem1, elem2 = elem2, sum
    return sum

for n in range(1, 10): # testing
    print(n, "->", fib(n))

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来源于思科网络学院-课程名称：PCAP-Python编程要点-模块4，第4.1.5.7章，题为“创建函数|斐波那契数”。

在Python中，

a，b=c，d

表示“设置

a=c

和

b=d

”

这句话几乎和说的一样

elem1 = elem2
elem2 =  sum

唯一的区别是它们“同时”发生。这可能很重要。例如，如果我写：

x, y = y, x

它交换元素，而

x = y
y = x 

---

没有。

对于代码elem1，elem2=elem2，sum的意思是：elem1=elem2，elem2=sum，或者您可以将其解释为i，j=1,10，其中i=1和j=10

elem1 = elem2 = 1 # Set the first two numbers of the fib sequence to 1
sum = 0
for i in range(3, n + 1): # if n is 3, we get -- for i in range(3,4), where i will only be 3 -- if n is 4, i will be 3, then 4 etc. 
    sum = elem1 + elem2 # First find the third number of the sequence, and during other iteration, the third will be shifted to the fourth, fifth, etc.
    elem1, elem2 = elem2, sum # Now shift the variables so the first number is the second, and the second is the sum(third) During each loop, we shift them again
return sum

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您应该阅读有关python解包的内容。“elem1逗号elem2等于elem2逗号和”更准确地说，

=

应该表示为“已分配”。这将帮助您避免在编程中混淆

=

和

=

。

x = y
y = x 

elem1 = elem2 = 1 # Set the first two numbers of the fib sequence to 1
sum = 0
for i in range(3, n + 1): # if n is 3, we get -- for i in range(3,4), where i will only be 3 -- if n is 4, i will be 3, then 4 etc. 
    sum = elem1 + elem2 # First find the third number of the sequence, and during other iteration, the third will be shifted to the fourth, fifth, etc.
    elem1, elem2 = elem2, sum # Now shift the variables so the first number is the second, and the second is the sum(third) During each loop, we shift them again
return sum