python - Why is np.where faster than pd.apply

Question

Sample code is here

import pandas as pd
import numpy as np

df = pd.DataFrame({'Customer' : ['Bob', 'Ken', 'Steve', 'Joe'],
                   'Spending' : [130,22,313,46]})

#[400000 rows x 4 columns]
df = pd.concat([df]*100000).reset_index(drop=True)

In [129]: %timeit df['Grade']= np.where(df['Spending'] > 100 ,'A','B')
10 loops, best of 3: 21.6 ms per loop

In [130]: %timeit df['grade'] = df.apply(lambda row: 'A' if row['Spending'] > 100 else 'B', axis = 1)
1 loop, best of 3: 7.08 s per loop

Question taken from here: https://stackoverflow.com/a/41166160/3027854

Answer 1

I think np.where is faster because use numpy array vectorized way and pandas is built on this arrays.

df.apply is slow, because it use loops.

vectorize operations are the fastest, then cython routines and then apply.

See this answer with better explanation of developer of pandas - Jeff.