Pandas - changing columns

import pandas as pd

df = pd.DataFrame({
   'Options': [ [], ["HELLO"], [], ["WORLD", "!!!"] ],
   'Product code': [ "A", "B", "C", "D" ]
})

def unnest(df, col, col2,reset_index=False):
   for item in df[col]:
        if len(item) == 0:
            item.append('[]')

   col_flat = pd.DataFrame([[i, x]
                      for i, y in df[col].apply(list).iteritems()
                          for x in y ], columns=['I', col]
                           )

   col_flat = col_flat.set_index('I')

   df = df.drop(col, 1)
   df = df.merge(col_flat, left_index=True, right_index=True)

   if reset_index:
       df = df.reset_index(drop=True)

   df['item_group_id'] = df['Product code']

   df['Product code'] += df[col].apply(lambda val: '' if val == '[]' else '-' + val)

   return df

print(df)

df = unnest(df,'Options','product code')

print(df)

# before
'''
        Options Product code
0            []            A
1       [HELLO]            B
2            []            C
3  [WORLD, !!!]            D
'''

# after

'''
  Product code Options item_group_id
0            A      []             A
1      B-HELLO   HELLO             B
2            C      []             C
3      D-WORLD   WORLD             D
3        D-!!!     !!!             D
'''