Setting default_factory of defaultdict to key

[John J Posner]

At the beginning of this thread, Ian Kelly said:

     Not with defaultdict, but you can subclass dict and provide a
     __missing__ method directly

To emphasize, you don't need to subclass "defaultdict" -- you need only subclass "dict" itself:

class MyDict(dict):
     def __missing__(self, key):
         self[key] = key
         return key

md = MyDict()
md[1] = 111
_ = md[2]
_ = md["another key"]


## md now looks like this:  {1: 111, 2: 2, 'another key': 'another key'}

The defaultdict documentation is confusing on this point. A *long* time ago, I filed Bug 9536 to improve the doc, but the fix hasn't bubbled to the surface yet.