How to assign values to multiple non existing columns in a pandas dataframe?

Question

So what I want to do is to add columns to a dataframe and fill them (all rows respectively) with a single value.

import pandas as pd
import numpy as np

df          


        

Answer 1

For me working:

df[["C","D"]] = pd.DataFrame([arr], index=df.index)

Or join:

df = df.join(pd.DataFrame([arr], columns=['C','D'], index=df.index))

Or assign:

df = df.assign(**pd.Series(arr, index=['C','D']))

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print (df)
   A  B  C  D
0  1  2  7  8
1  3  4  7  8