I would do:
```python
possible_abundances={'coronal': 'cor', 'photospheric': 'pho',}
if possible_abundances.has_key(abundances):
    abund = possible_abundances[abundances]
else: 
    raise ValueError("the possible values are either " + " or ".join(possible_abundances.keys()))
```
This way, if we start to add more abundances we have just to change the possible_abundances dictionary.