how to combine two columns to one in pandas code example

Example 1: merge two columns name in one header pandas

df['A'] = df[a_cols].apply(' '.join, axis=1)

Example 2: pandas merge two columns from different dataframes

#suppose you have two dataframes df1 and df2, and 
#you need to merge them along the column id
df_merge_col = pd.merge(df1, df2, on='id')