It's just a question of style/readability so please consider it just a suggestion, but how about something like:
```
# Line below could also be: params = [('ellps', self.ellipse), ('datum', self.datum), ...]
params = {'ellps': self.ellipse, 'datum': self.datum, ...} 
proj4_params = dict((k, v) for k, v in params if v is not None)
return proj4_params
```