This idiom for default values is *slightly* dangerous. The function doesn't modify ``sbs`` anywhere but future commits may well modify it to do so. At which point there'll be strange behaviour. (See [1].) I'd prefer to set ``sbs=None`` as the default and add the line:

```python
sbs = sbs if sbs if not None else np.arange(6)
```

To the head of the function.

[1] http://effbot.org/zone/default-values.htm