Standardize by group with pandas

Introduction

I was processing data for machine learning with pandas, I wanted to standardize by group of some columns rather than standardize as a whole. However, there was a scene where standardization was not necessary for the group name, but the standardization process was desired while retaining the group name. It's just a memo.

Execution environment

pandas = 0.25.3 numpy = 1.18.0

Code to standardize by group in pandas

Standardize columns for each class name in a table like the one below

class a b c
a 1.0 2.0 3.0
a 4.0 5.0 6.0
b 7.0 8.0 9.0
b 10.0 11.0 12.0

import pandas as pd
import numpy as np

# make data set
df = pd.DataFrame(np.arange(12).reshape(4, 3),
                  columns=['col_0', 'col_1', 'col_2'],
                  index=['row_0', 'row_1', 'row_2','row_3'])
df["class"] = ["a", "a", "b", "b"]

# Standardization for each group
class_ = df[["class"]]
class_names = df.groupby("class").groups.keys()
for name in class_names:
     df_tmp = df[(df['class'] == name)].drop(columns=['class'])
     df[(df['class'] == name)] =  (df_tmp - df_tmp.mean()) /df_tmp.std()
df["class"] = class_

First post. .. It's just a memo. Please let me know if there is a better way.

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