I'm assuming this was already discussed, but it should be possible to leave these as properties of the object. My 2-cents here is that

```python
>>> DNASequence.iupac_degeneracies['B']
```

is cleaner than:

```python
>>> DNASequence.iupac_degeneracies()['B']
```

And I believe this could be accomplished with:

```python
class BiologicalSequence(Sequence):
    # ... snip ...
    @classmethod
    def alphabet_getter(cls):
        return foo

    @property
    def alphabet(self):
        return self.alphabet_getter()
```

...though that is a bit dirty to look at