Two way/reverse map

You can create your own dictionary type by subclassing dict and adding the logic that you want. Here's a basic example:

class TwoWayDict(dict):
    def __setitem__(self, key, value):
        # Remove any previous connections with these values
        if key in self:
            del self[key]
        if value in self:
            del self[value]
        dict.__setitem__(self, key, value)
        dict.__setitem__(self, value, key)

    def __delitem__(self, key):
        dict.__delitem__(self, self[key])
        dict.__delitem__(self, key)

    def __len__(self):
        """Returns the number of connections"""
        return dict.__len__(self) // 2

And it works like so:

>>> d = TwoWayDict()
>>> d['foo'] = 'bar'
>>> d['foo']
'bar'
>>> d['bar']
'foo'
>>> len(d)
1
>>> del d['foo']
>>> d['bar']
Traceback (most recent call last):
  File "<stdin>", line 7, in <module>
KeyError: 'bar'

I'm sure I didn't cover all the cases, but that should get you started.

In your special case you can store both in one dictionary:

relation = {}
relation['Alice'] = 'Bob'
relation['Bob'] = 'Alice'

Since what you are describing is a symmetric relationship. A -> B => B -> A