exec将函数添加到类中 [英] exec to add a function into a class

问题描述

因此，我研究了类似的问题，并且找到了一些解决方案，但是我还不太清楚如何做到这一点.

So I've looked at similar questions, and I've found some solutions to this, but I can't quite figure out how to do this.

我想做的是从字符串向类添加方法.我可以使用setattr()方法来执行此操作，但这不会让我在其他方法中将self用作属性.这是一个示例:(我为变量名表示歉意，在构思一个想法时总是使用yolo)

What I'm trying to do is add a method to a class from a string. I can do this with the setattr() method, but that won't let me use self as an attribute in the extra method. Here's an example: (and I apologize for the variable names, I always use yolo when I'm mocking up an idea)

class what:
    def __init__(self):
        s = 'def yolo(self):\n\tself.extra = "Hello"\n\tprint self.extra'
        exec(s)
        setattr(self,"yolo",yolo)

what().yolo()

返回此:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: yolo() takes exactly 1 argument (0 given)

，如果s = 'def yolo():\n\tself.extra = "Hello"\n\tprint self.extra' 然后我得到这个结果:

and if s = 'def yolo():\n\tself.extra = "Hello"\n\tprint self.extra' then I get this result:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 2, in yolo
NameError: global name 'self' is not defined

从本质上讲，这意味着我无法为类动态创建方法，我知道这是一种不好的做法，而且是非Python的，因为这些方法将无法访问该类其余部分可以访问的变量.

This essentially means that I cannot dynamically create methods for classes, which I know is bad practice and unpythonic, because the methods would be unable to access the variables that the rest of the class has access to.

感谢您的帮助.

推荐答案

您必须将函数绑定到类实例，才能将其转换为方法.可以将其包装在 types.MethodType 中:

You have to bind your function to the class instance to turn it into a method. It can be done by wrapping it in types.MethodType:

import types

class what:
    def __init__(self):
        s = 'def yolo(self):\n\tself.extra = "Hello"\n\tprint self.extra'
        exec(s)
        self.yolo = types.MethodType(yolo, self)

what().yolo()

在旁注中，为什么在这种情况下您甚至需要exec?你也可以写

On a side note, why do you even need exec in this case? You can just as well write

import types

class what:
    def __init__(self):
        def yolo(self):
            self.extra = "Hello"
            print self.extra

        self.yolo = types.MethodType(yolo, self)

what().yolo()

编辑:出于完整性考虑，人们可能更喜欢通过

for the sake of completeness, one might prefer a solution through the descriptor protocol:

class what:
    def __init__(self):
        def yolo(self):
            self.extra = "Hello"
            print self.extra

        self.yolo = yolo.__get__(self)

what().yolo()

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