Python：在字典中将不等式传递给评估 [英] Python: Pass inequality as string in dict for evaluation

问题描述

我需要在函数中将不等式传递给函数进行评估。如果作为字符串传递，是否有评估不平等的方法？或者我必须传递一个不等式的表示并使用if / else语句来生成符号？解决方案

你的问题有点儿含糊不清，但它听起来像你想评估一个包含表达式的字符串（如 x> 5 ）。而不是这样做，这是不必要的复杂，并可能是一个安全隐患，只需定义一个函数，无论是传统的方式或使用lambda。

  def gt5（x）：
 return x> 5 
  

或

  gt5 = lambda x：x> 5 
  

这些都是等价的;现在你可以传递 gt5 ，不管你喜欢什么时候，只要把它叫做

  y = 6 
如果gt5（y）：
 ... 
  

由于杰拉特的

a>答案表明，操作员模块可能对这样的事情也很有用。 / p>

---

现在我知道您正在处理用户字符串，我肯定会建议创建一个将字符串映射到函数的字典。 （也许这就是你的标题中的含义？）将用户域字符串传递给 getattr 在很多方面看起来很糟糕。如果你想添加一个不符合 operator 属性的字符串，会发生什么？如果用户传递一个对应于私有属性的字符串会发生什么？最好创建一个将字符串映射到函数的自定义字典。字典允许你指定你想接受的字符串。

  func_dict = {
'lt'：operator.lt，
'gt'：operator.gt ，
'nand'：lambda x，y：not（x and y）
} 
  

通过使用 getattr + 运算符，您仍然可以通过字符串列表构建字典你想接受。例如：

  func_dict = dict（（s，getattr（operator，s））for ['lt'，' gt']）
  

然后更新 func_dict like所以：

  custom_dict = {
'nand'：lambda x，y：not（x and y）
 
 
 func_dict.update（custom_dict）
  

从那里你可以轻松调用字典中的函数，如下所示：

 >>> func_dict ['lt']（5，7）
 True 
  

I need to pass inequalities to a function for evaluation within the function. Is there a way to evaluation the inequality if passed as a string? Or must I pass a representation of the inequality and use if/else statements to generate the sign?

解决方案

Your question is a little vague, but it sounds like you want to evaluate a string containing an expression (such as x > 5). Rather than doing that, which is unnecessarily complex, and potentially a security hazard, just define a function, either in the conventional way or using lambda.

def gt5(x):
    return x > 5

or

gt5 = lambda x: x > 5

These are both equivalent; now you can pass around gt5 however you like, and when the time comes, you simply call it

y = 6
if gt5(y):
    ...

As Gerrat's answer suggests, the operator module may also be useful for things like this.

---

Now that I know you are processing user strings, I would definitely suggest creating a dictionary that maps strings to functions. (Perhaps that's what you meant in your title?) Passing userland strings into getattr seems bad in a number of ways. What happens if you want to add a string that doesn't correspond to an attribute of operator? What happens if the user passes in a string corresponding to a private attribute? Better to create a custom dictionary mapping strings to functions. A dict allows you to specify just those strings you want to accept.

func_dict = {
    'lt'   : operator.lt,
    'gt'   : operator.gt,
    'nand' : lambda x, y: not (x and y)
}

You could still save yourself work by using getattr + operator to build the dict from a list of strings that you want to accept. Something like:

func_dict = dict((s, getattr(operator, s)) for s in ['lt', 'gt'])

Then update func_dict like so:

custom_dict = {
    'nand' : lambda x, y: not (x and y)
}

func_dict.update(custom_dict)

From there you can easily call functions in the dict like so:

>>> func_dict['lt'](5, 7)
True

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