**IMPORTANT NOTE:** The pandas-ply package is now called `pandas_ply` rather than just `ply`. You should write:
python
from pandas_ply import A, B, C
rather than the older
python
from ply import A, B, C
Sorry for the trouble, but the [other ply](https://pypi.python.org/pypi/ply/) beat us to the name by 13 years.