Advantages of UserDict class in Python

What are advantages of using UserDict class?

I mean, what I really get if instead of

class MyClass(object):
    def __init__(self):
        self.a = 0
        self.b = 0
...
m = MyClass()
m.a = 5
m.b = 7

I will write the following:

class MyClass(UserDict):
    def __init__(self):
        UserDict.__init__(self)
        self["a"] = 0
        self["b"] = 0
...
m = MyClass()
m["a"] = 5
m["b"] = 7

Edit: If I understand right I can add new fields to an object in a runtime in both cases?

m.c = "Cool"

and

m["c"] = "Cool"

回答1:

UserDict.UserDict has no substantial added value since Python 2.2, since, as @gs mention, you can now subclass dict directly -- it exists only for backwards compatibility with Python 2.1 and earlier, when builtin types could not be subclasses. Still, it was kept in Python 3 (now in its proper place in the collections module) since, as the docs now mention,

The need for this class has been partially supplanted by the ability to subclass directly from dict; however, this class can be easier to work with because the underlying dictionary is accessible as an attribute.

UserDict.DictMixin, in Python 2, is quite handy -- as the docs say,

The module defines a mixin, DictMixin, defining all dictionary methods for classes that already have a minimum mapping interface. This greatly simplifies writing classes that need to be substitutable for dictionaries (such as the shelve module).

You subclass it, define some fundamental methods (at least __getitem__, which is sufficient for a read-only mapping without the ability to get keys or iterate; also keys if you need those abilities; possibly __setitem__, and you have a R/W mapping without the ability of removing items; add __delitem__ for full capability, and possibly override other methods for reasons of performance), and get a full-fledged implementation of dict's rich API (update, get, and so on). A great example of the Template Method design pattern.

In Python 3, DictMixin is gone; you can get almost the same functionality by relying on collections.MutableMapping instead (or just collections.Mapping for R/O mappings). It's a bit more elegant, though not QUITE as handy (see this issue, which was closed with "won't fix"; the short discussion is worth reading).

回答2:

Subclassing the dict gives you all the features of a dict, like if x in dict:. You normally do this if you want to extend the features of the dict, creating an ordered dict for example.

BTW: In more recent Python versions you can subclass dict directly, you don't need UserDict.