Python wrapper class

Question

I've been exposed to python for about 2 years and I have never used or come across a wrapper class until today when I see something like this:

class Wrapper(object):
　　def list_to_str(self, my_list):
　　　　if self.mode == 'convert':
　　　　　　return str(my_list)

　　def get_tree(self, my_list)
    　　my_tree = self.list_to_tree(my_tree)
　　　  return my_tree

class RealClass(Wrapper):

　　def __init__(self, mode):
　　　　self.mode = mode
　　
　　def list_to_tree(self, list):
       ""convert list to tree""
       return tree

I was quite annoyed by this code and personally find it really ugly.

However, because I have not been exposed to wrapper class in production environment (or any other environment) I'm not sure if the above practice is find.

Please tell me if the ^ is OK, and if not, please tell me how should I re-factor these 2 classes (completely remove wrapper?) or provide me some resources.

So far I looked at the python documentation and some other posts related to the wrapper class, but it seems that all the _ getattribute _ and stuff like that doesn't really apply here.

This is basic class inheritance. You should read about object-oriented programming in order to understand what's going on. Also, something like this is very common and an okayish use of the dynamic nature of python in order to provide an abstract implementation. — Holger Just
– Holger Just, Commented Mar 4, 2015 at 16:47
@HolgerJust I understand why this wrapper class works and stuff like that, but what i don't know if it's fine to, say, not having an init in the wrapper — user1948847
– user1948847, Commented Mar 4, 2015 at 16:52
Why would Wrapper need an init? Do you ever instantiate the Wrapper? — Ionut Hulub
– Ionut Hulub, Commented Mar 4, 2015 at 16:59
What might confuse you is that the class named Wrapper is not an actual wrapper in your case. It is a parent class to RealClass. This, you probably only create RealClass instances which inherit some of their behavior from the Wrapper class. — Holger Just
– Holger Just, Commented Mar 4, 2015 at 17:10
In Python, the concept of decorators (or wrappers) is something entirely different. In your example, you don't use a decorator. — Holger Just
– Holger Just, Commented Mar 4, 2015 at 17:12

tdelaney · Accepted Answer · 2015-03-04 17:12:54Z

The problem is that your example isn't a wrapper class at all. It could either be viewed as an abstract base class or as a mix-in. In either case, the idea is to have a class that implements some functionality but isn't intended to stand on its own. Its common for these types of classes to skip initialization and just assume that a real implementing class sets up the object properly.

Generally, classes aren't required to have an __init__ but its very useful for setting up instance variables and so most classes have them.

In your case, the mis-named Wrapper assumes that an implementor will define self.mode and self.list_to_tree. If that contract is satisfied, it does list_to_str and get_tree for you. There could be multiple implementors who have different views of what mode and list_to_tree are, but they would all get the same functionality from the base class.

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