Why aren't Python dicts unified?

Question

After reading this question, I noticed that S. Lott might have liked to use an “ordered defaultdict”, but it doesn't exist. Now, I wonder: Why do we have so many dict classes in Python?

• dict
• blist.sorteddict
• collections.OrderedDict
• collections.defaultdict
• weakref.WeakKeyDictionary
• weakref.WeakValueDictionary
• others?

Why not have something like this,

dict(initializer=[], sorted=False, ordered=False, default=None, 
     weak_keys=False, weak_values=False)

that unifies everything, and provides every useful combination?

Answer 1

One issue is that making this change would break backward-compatibility, due to this type of constructor usage that exists now:

>>> dict(one=1, two=2)
{'two': 2, 'one': 1}