如何将要由子类实现的空函数包装到tryexcept块中？

Question

我正在创建一个抽象的网络爬虫类爬虫。 此类只能与实现某些方法的子类一起使用。在

（删除了init方法，因为它是无关的）

class Crawler():
    def get_image_source_url(self, image_page_soup):
        return NotImplementedError("method get_image_source_url must be implemented")

    def get_image_thumbnail_url(self, image_page_soup):
        return NotImplementedError("method get_image_thumbnail_url must be implemented")

    def get_tags_container(self, image_page_soup):
        return NotImplementedError("method get_tags_container must be implemented")

它们在子类中是这样实现的：

^{pr2}$

我想把函数包装在try-except块中，不管它是如何实现的。所以最终的结果是：（伪代码）

class Crawler():
    def get_image_source_url(self, image_page_soup):
        if not_implemented:
            return NotImplementedError("method get_image_thumbnail_url must be implemented")
        else:
            try:
            whatever the child class chooses to do
            except Exception:
            handle exception however i decide in the parent class

我不知道如何检查它是否已实现，以及如何在不覆盖所有子类的情况下包装函数，我是否清楚我要实现的目标？在

Answer 1

^{Note: This post contains two different implementation techniques to allow for what you want.}

---

间接求解

解决此问题的最简单方法是重构代码，使子类不直接重写公共接口使用的函数。在

相反，直接在基类中提供公共功能，并使子级重写所述函数的“worker”（实现细节），该函数稍后由从外部调用的函数调用。在

---

示例实现

class Base (object):
    def get_message (self):
        try:
            return self.get_message_impl ()
        except Exception as detail:
            print ("error:", detail)    
        return None

    def get_message_impl (self):
        raise Exception ("Not Implemented")

^{pr2}$

class Bar (Base):
    def get_message_impl (self):
        raise Exception ("Bar.get_message_impl always fails!")

f = Foo ()
b = Bar ()

f_msg = f.get_message ()
b_msg = b.get_message ()

print ("f_msg:", f_msg)
print ("b_msg:", b_msg)

_输出

error: Bar.get_message_impl always fails!
f_msg: Hello World
b_msg: None

---

---

选择保护

如果您希望保持重写基类中呈现的公共功能的可能性，同时在以后仍然能够轻松调用函数的“protected”版本，则可以创建一个简单的包装器，如下所示：

class Base (object):
    class Protected (object):
        def __init__ (self, target):
            self.target = target

        def get_message (self):
            try:
                return self.target.get_message ()
            except Exception as detail:
                print ("error:", detail)
            return None

    def __init__ (self):
        self.protected = self.Protected (self)

    def get_message (self):
        raise Exception ("Not Implemented")

class Foo (Base):
    def get_message (self):
        return "Hello World";

class Bar (Base):
    def get_message (self):
        raise Exception ("Bar.get_message_impl always fail!")

f = Foo ()
b = Bar ()

f_msg = f.protected.get_message () # protected from failure
b_msg = b.protected.get_message () # protected from failure

b_msg = b.get_message ()           # will raise exception