How many function calls when I use pandas DataFrame groupby and then apply user-defined function?

Question

I have a pandas DataFrame:

df = pd.DataFrame({'A':['a', 'a', 'a', 'a', 'a'], 'B':[1, 2, 3, 4, 5]})

and a user-defined function:

def f(df):
    return df.max() - df.min()

When I run the following code:

df.groupby('A').apply(f)

How many times the function f will be called? I tested this code, and found f will be called 4 times, could anybody explain this?

Answer 1

It should be called ngroup times , if you have two group in A , it will be called twice. Also , you can using np.ptp

df.groupby('A').B.apply(np.ptp)
Out[52]: 
A
a    4
Name: B, dtype: int64