Osx

@albertogaitan this is for you. Check out my comments here.
In previous versions of this code `HumanFinder` was defined as:

```python
class HumanFinder:
  pass
```
That is the old style of creating a new class. It's not great because it breaks things like `isinstance`, `super` and friends. Instead try:

```python
class HumanFinder(object):
  pass
```