You didn't mention the fancy indexing capabilities of dataframes, e.g.:
>>> df = pd.DataFrame({"class":[1,1,1,2,2], "value":[1,2,3,4,5]})
>>> df[df["class"]==1].sum()
class 3
value 6
dtype: int64
>>> df[df["class"]==1].sum()["value"]
6
>>> df[df["class"]==1].count()["value"]
3
You could replace df["class"]==1by another condition.