多次元配列を一次元配列になおすravel関数

import pandas as pd
import numpy as np

df = pd.DataFrame([['本', "2022/1", 3000], ['食品', "2022/1", 70000], ['衣料品', "2022/1", 1000]])
df.columns = ['カテゴリ', '年月', '購入金額']

a = df[['カテゴリ']].values
print(a)
print(type(a))

b = np.ravel(a)
print(b)
print(type(b))