This is not of first importance here, but just to let you know, Informations such as important properties should be in notes section.

```python
"""
A dummy cost used for testing.

Notes
-----
Important properties:
has a expr method which takes a model and a 
dataset and returns as cost the mean squared 
difference between the data and the models'
output using that data.
"""
```
You should do it here to help you remember next time when it will be important :)