Merge

Merge by Index

pd.merge(train_1, train_2, left_index=True, right_index=True)

Merge by a Column

 pd.merge(left=train_1, right=train_2, how='left', on=['column'])

Merge by Columns

 pd.merge(left=train_1, right=train_2, how='left', on=['column_1', 'column_2'])