Question

I’m currently looking through the “Dive Into Python” to pick up the language and I was a bit confused about chapter 5’s example 5.10. UserDict Normal Methods

Example 5.10. UserDict Normal Methods

def copy(self):
    if self.__class__ is UserDict:
        return UserDict(self.data)
    import copy
    return copy.copy(self)

where data is a dictionary.

I notice that if the class is a UserDict type, then it returns UserDict(self.data). What I’m confused about is, why do you need to return UserDict(self.data) instead of just returning self.data? Isn’t self.data a dictionary which you can return?

If someone can explain the difference between returning UserDict(self.data) and self.data, I would appreciate it greatly.

Answer 1

In order to yield a copy, you should use UserDict(self.data).

Why we use UserDict(self.data) instead of self.data is that it returns new instance which is same class with self.

If you return only self.data then you didn’t make a copy. Because it returns a dict, not an instance of UserDict. If you want to produce a copy, you should use UserDict(self.data).