Ahh, know I get it. You mean:
```python
if len(assignments) > 1:
  assign_names = _("%s and %s") % (", ".join(assignments[:-1]), assignments[-1])
elif len(assignments) == 1:
  assign_names = assignments[0]
else:
  assign_names = ''
```
I was irritated from the ```''.join(assignment[-1:])``` which is the same as ```assignment[-1]``` if there is at least one item in the list.

Maybe it would help the readability to use more then one line.