Answers for "Which parameter of the merge() function specifies the type of join to be performed on the dataframes?"

merge two columns name in one header pandas

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

merge dataframe pandas

>>> df1.merge(df2, left_on='lkey', right_on='rkey')
  lkey  value_x rkey  value_y
0  foo        1  foo        5
1  foo        1  foo        8
2  foo        5  foo        5
3  foo        5  foo        8
4  bar        2  bar        6
5  baz        3  baz        7