how to copy a column from one dataframe to another code example

Example: pandas copy data from a column to another

df = pd.DataFrame()

df['X'] = [0, 3, 1, 1, 2, 2, 3, 3, 1, 2]
df['Y'] = [111.0, np.nan, np.nan, 112, 113, np.nan, 114, 115, np.nan, 116]

df['Y'] = df['Y'].fillna(df['X'])

print(df)