LUN-1813

Even though I disagree, PEP 8 advises against named lambdas.
http://legacy.python.org/dev/peps/pep-0008/#programming-recommendations
For the is_site_allowed you may also use a generator expression:
```python
is_site_allowed = next((site
                        for site in self.sites 
                        if site.id == self._current_site.id), None)
```