Python how to combine two columns of a dataframe into a single list?

The underlying numpy array is organized array([[row1], [row2], ..., [rowN]]) so we can ravel it, which should be very fast.

df[['data1', 'data2']].to_numpy().ravel().tolist()#[20, 120, 30, 456, 40, 34]

Because I was interested: Here are all the proposed methods, plus another with chain, and some timings for making your output from 2 columns vs the length of the DataFrame.

import perfplotimport pandas as pdimport numpy as npfrom itertools import chainperfplot.show(    setup=lambda n: pd.DataFrame(np.random.randint(1, 10, (n, 2))),     kernels=[        lambda df: df[[0, 1]].to_numpy().ravel().tolist(),        lambda df: [x for i in zip(df[0], df[1]) for x in i],        lambda df: [*chain.from_iterable(df[[0,1]].to_numpy())],        lambda df: df[[0,1]].stack().tolist()  #  proposed by @anky_91    ],    labels=['ravel', 'zip', 'chain', 'stack'],    n_range=[2 ** k for k in range(20)],    equality_check=np.allclose,      xlabel="len(df)")

That doesn't work since it won't add by same index, use the below list comprehension:

print([x for i in zip(df['data1'], df['data2']) for x in i])