Python: Pandas Dataframe AttributeError: 'numpy.ndarray' object has no attribute 'fillna'

Question

Since I am creating a dataframe, I don't understand why I am getting an array error.

M2 = df.groupby(['song_id', 'user_id']).rating.mean().unstack()
M2 = np.maximum(-1, (M - 3).fillna(0) / 2.)  # scale to -1..+1  (treat "0" scores as "1" scores)
M2.head(2)

AttributeError: 'numpy.ndarray' object has no attribute 'fillna'

Answer 1

(M - 3) is getting interpreted as a numpy.ndarray. This implies that M is defined somewhere as a numpy.ndarray. Test it out by running:

print type(M)