Python如何在一个if语句中检查变量是否存在及其长度？

发布于 2024-12-05 18:17:04 字数 283 浏览 0 评论 0原文

这是我的情况：

if var:
    if len(var) == 5:
        do something...
else:
    do the same thing...

为了避免重复同一段代码，我想将这 2 个 if 条件合并为一个。但如果 var 是 None，我无法检查它的长度......知道吗？我想要这样的东西：

if var and len(var) == 5:
    do something...

原文

Here's my situation:

if var:
    if len(var) == 5:
        do something...
else:
    do the same thing...

To avoid repeating the same piece of code, I would like to combine those 2 if conditions, in one. But if var is None, I can't check its length... Any idea?
I would like something like this:

if var and len(var) == 5:
    do something...

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萌逼全场 2024-12-12 18:17:04

你尝试过吗？它有效：

if var and len(var) == 5:
    ....

如果 LHS 为 false，则 and 运算符不会计算 RHS。试试这个：

>>> False and 1/0
False
>>> True and 1/0
ZeroDivisionError: division by zero

Did you try that? It works:

if var and len(var) == 5:
    ....

The and operator doesn't evaluate the RHS if the LHS is false. Try this:

>>> False and 1/0
False
>>> True and 1/0
ZeroDivisionError: division by zero

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纵山崖 2024-12-12 18:17:04

您在问题中编写的代码在两种情况下执行语句：

var 计算结果为 true，长度为 5
var 计算结果为 false（例如，它是 < code>None 或 False）

您可以将它们组合成一个条件，如下所示：

if not var or len(var) == 5:
    ...

其他一些答案建议的条件，var and len(var) == 5，仅评估为True 在第一种情况下，而不是在第二种情况下。不过，我要说的是，您选择的两种情况是一种不寻常的组合。您确定您不打算仅在变量具有非假值且长度为 5 时才执行语句吗？

此外，正如 6502 在评论中所写，函数正是针对这种情况，即制作可重用的代码块。所以另一个简单的解决方案是

 def f():
     ....
 if var:
     if var.length == 5:
         f()
 else:
     f()

The code as you've written it in the question executes the statements in two cases:

var evaluates to true and has a length of 5
var evaluates to false (e.g. it's None or False)

You can combine those into one conditional as follows:

if not var or len(var) == 5:
    ...

The conditional that some other answers are suggesting, var and len(var) == 5, only evaluates to True in the first case, not the second. I will say, though, that the two cases you've chosen are kind of an unusual combination. Are you sure you didn't intend to execute the statements only if the variable has a non-false value and has a length of 5?

Additionally, as 6502 wrote in a comment, functions are intended for exactly this case, namely making reusable code blocks. So another easy solution would be

 def f():
     ....
 if var:
     if var.length == 5:
         f()
 else:
     f()

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梦中的蝴蝶 2024-12-12 18:17:04

好吧，例外在这里似乎完全没问题。

try:
    if len(var) == 5:
        do_something()
except TypeError:
    pass

您说 var 可以为 null，因此会抛出 TypeError 异常。然而，如果事实上 var 可能根本不存在，那么您应该捕获 NameError 异常。

Well, exceptions seem to be perfectly fine here.

try:
    if len(var) == 5:
        do_something()
except TypeError:
    pass

You're saying that var can be null, therefore a TypeError exception would be thrown. If, however, in fact var can be non-existant at all, you should catch for NameError exception.

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