this is a situation where the list comprehension seems cleaner to me:

```python
        data = []
        for md in bt.ObservationMetadata:
            tmp = []
            for k,v in md[md_key].iteritems():
                tmp.append('%s_%s' % (k,v))
            data.append(' '.join(tmp))
``` 
vs
```python
data = [' '.join(['%s_%s' % (k,v) for k,v in md[md_key].items()]) for \
            md in bt.ObservationMetadata]
```