Because http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument

I am thinking whether it can do something bad in this code. It seems that it's safe here as it does not modify the items of the self.locators anywhere. But when in doubt, it can be replaced with simple

```python
def __init__(self, locators=None, title=None, identifying_loc=None):
...
self.locators = locators or {}
```