Idiomatic way to get array of column values in a pandas dataframe

Question

I have a dataframe where I want to get a single array of all of the values in the 'a' column, which is part of a multi-index dataframe. The code below works, but it is hard to read, write, and think about. Is there a more idiomatic way to express the same idea?

import numpy as np
import pandas as pd

x = pd.DataFrame({'a': [1, 2, 3], 'b': [1, 2, 3]})
y = pd.DataFrame({'a': [11, 12, 13], 'b': [21, 22, 23]})

df = pd.concat({'x': x, 'y': y}, axis=1)

x = np.concatenate(df.loc[:, (slice(None), 'a')].values)

df:
   x      y
   a  b   a   b
0  1  1  11  21
1  2  2  12  22
2  3  3  13  23

x:
[ 1 11  2 12  3 13]

Answer 1

Approach 1: xs

np.ravel(df.xs('a', level=1, axis=1))

Approach 2: stack

list(df.stack(0)['a'])

Result

[1, 11, 2, 12, 3, 13]