测试通过函数传递到if语句的Truthy或Falsy参数 [英] Testing Truthy or Falsy arguments passed through a function into an if statement

问题描述

我也在这一点上画了一个空白。如果有人能帮助我理解为什么我的代码没有打印出预期的输出，我将不胜感激。

I am drawing a blank on this one too. Rather than provide an answer, I would appreciate if someone could help me understand why my code is not printing the expected output:

def bool_to_str(bval):
    if bval is True:
        mytest = 'Yes'
    else:
        mytest = 'No'
    return mytest

预期产出：

>>>bool_to_str([1, 2, 3])
'Yes'
>>>bool_to_str(abcdef)
'Yes'

实际输出的是什么：

>>>bool_to_str([1, 2, 3])
'No'
>>>bool_to_str(abcdef)
'No'

请帮助我理解我做错了什么。我认为函数需要测试参数的实际真值，但我不明白我错过了什么。

Please help me to understand what I did wrong. I think that the function needs to test the actual truth value of the argument, but I don't understand what I'm missing.

推荐答案

检查参考相等 ，而不是真实性。现在很明显 [1,2,3] （这是一个列表对象）没有指向 True object （ bool 对象）。很难说这里没有定义的 abcdef 指向 True 。但是既然你没有提供它，我会假设它指向不同的东西。

The is checks reference equality, not truthiness. Now clearly [1,2,3] (which is a list object) does not point to the True object (which is bool object). It is hard to say if abcdef which is not defined here points to True. But since you do not provide it, I gonna assume it points to something different.

只有 bool_to_str（真）或 bool_to_str（< expr>）其中< expr> 评估为 bool 即 True 将导致'是'（ bool s是单身人士，因此所有 True s都是同一个对象。）

Only bool_to_str(True) or bool_to_str(<expr>) where <expr> evaluates to a bool that is True will result in 'Yes' (the bools are singletons, so all Trues are the same object).

关键是，为了检查< expr> 的真实性，如果< expr>，只需写下 ;： 即可。所以在你的情况下它应该是：

The point is that in order to check the truthness of <expr>, simply write if <expr>:. So in your case it should be:

if bval:

你也可以 - 虽然我建议反对它，用 bool（..）明确检查真相并检查参考相等如：

You can also - although I advise against it, check the truthness explicitly with bool(..) and check reference equality like:

if bool(bval) is True:

通常写不是一个好主意。仅当您要检查两个变量是否指向相同（即不 等效）对象，或者某些单个对象（如<$ c）时$ c> True ，无，（）等，这真的很有意义。

Usually it is not a good idea to write is. Only if you want to check if two variables point to the same (i.e. not equivalent) object, or for some singleton objects like True, None, (), etc. it makes really sense.

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