How do I solve the value error in Python?

for dataset in train_df:
    pd.to_numeric(dataset['Embarked'].map( {'S': 0, 'C': 1, 'Q': 2}))

TypeError                                 Traceback (most recent call last)
<ipython-input-41-9e474db5d734> in <module>
      1 for dataset in train_df:
----> 2     pd.to_numeric(dataset['Embarked'].map( {'S': 0, 'C': 1, 'Q': 2}))

TypeError: string indices must be integers