Changed .unique().shape[0] to .nunique()

[sursu]

I've recently learned how to do PRs.
So, here's a tiny change I'm proposing.

[CamDavidsonPilon]

cool, TIL about this feature

[CamDavidsonPilon]

There are unit test errors, and I think they caused by us carrying a multi-index around which just happens to have the (possibly redundant) id col - all comes full circle. Let me tie this into the issue #809, and once I decide on that topic, then this PR can be addressed.

[sursu]

The issue with MultiIndex comes from pandas apparently:

In JupyterLab if I run:

arrays = [[1, 1, 1, 2], ['red', 'blue', 'red', 'blue']]
mi = pd.MultiIndex.from_arrays(arrays, names=('number', 'color'))

mi.nunique()

I get the following error:

/opt/anaconda3/lib/python3.7/site-packages/pandas/core/dtypes/missing.py in _isna_new(obj)
    131     # hack (for now) because MI registers as ndarray
    132     elif isinstance(obj, ABCMultiIndex):
--> 133         raise NotImplementedError("isna is not defined for MultiIndex")
    134     elif isinstance(
    135         obj,

NotImplementedError: isna is not defined for MultiIndex

The comment there says it all.

I guess this PR can be closed.

[CamDavidsonPilon]

I'd like to keep it open, if you don't mind. It's a reminder to address this.