为什么这段代码会失败？在for循环中将变量分配给自身列表

Question

a = 'hello'
b = None
c = None
for x in [a, b, c]:
    if isinstance(x, (basestring, dict, int, float)) or x is None:
        x = [x]
a

returns'hello'，但预计 ['hello']。但是，这有效：

a = 'hello'
a = [a]
a

returns ['hello'].

Answer 1

让我们一次完成这一行；

a = 'hello'  # assigns 'hello' to a.
b = None  # etc...
c = None
for x in [a, b, c]:  # Creates list from the values in 'a' 'b' and 'c', then iterates over them.
    # This if statement is always True. x is always either a string, or None.
    if isinstance(x, (basestring, dict, int, float)) or x is None:
        x = [x]  # On the first iteration, sets variable to the expected ['hello']
        # After that though - it replaces it with [None]
a  # = 'hello' - you have not assigned anything to the variable except on the first line.

唯一设置为 ['hello'] 的变量是 x，它很快被 None 覆盖。如果您将 if 检查更改为排除 or x is None 并分配给 a 而不是 x，您将获得所需的结果。

还值得注意的是，列表 [a, b, c] 是在您启动 for 循环时创建的。在 for 循环中更改 a b 或 c 将无效 - 列表已经创建。

Answer 2

要做到这一点，首先您必须了解您有两个不同的引用，a 和 x（针对每个元素），以及列表 [a,b,c] 的引用，仅在 for 循环中使用，再也不会了。

为了实现你的目标，你可以这样做：

a = 'hello'
b = None
c = None
lst = [a, b, c] #here I create a reference for a list containing the three references above
for i,x in enumerate(lst):
    if isinstance(x, (str,dict, int, float)) or x is None: #here I used str instead basestring
        lst[i] = [x]

print(lst[0]) #here I print the reference inside the list, that's why the value is now showed modified

['hello']

但正如我所说，如果您执行 print(a) 它将再次显示：

'hello' #here i printed the value of the variable a, wich has no modification

因为你从来没有用它做过任何事情。

看看这个问题，了解更多关于参考文献的知识How do I pass a variable by reference?

Answer 3

其他答案都很棒。我认为，概括地说，列表元素是 immutable/hashable 所以 for 循环 returns 是一个副本，而不是对原始对象的引用。我早该发现这一点，但教我编码太久没睡觉！

我最终使用了这个：

def lst(x):
    if isinstance(x, (basestring, dict, int, float)) or x is None:
        x = [x]
    return x
[a, b, c] = map(lst, [a, b, c])

为什么这段代码会失败？在for循环中将变量分配给自身列表

Why does this code fail? assigning variable to list of itself in for loop

python

variables

reference

immutability

variable-assignment