Change Values in Dataframe with Values in Some Other Columns in Other Dataframe

I want to change values in my datframe

student = pd.DataFrame({'id': [1,2,3,4,5,6,7,8,9,10,],
                        'homeground': ['TOKYO','SOUTH KOREA','RIYADH','JAPAN','TOKYO','OSAKA','SAUDI ARABIA','SEOUL','','BUSAN']})

this is the master homegroud

hg = pd.DataFrame({'id_country':[1,2,2,3,3],
                   'country': ['Saudi Arabia','South Korea','South Korea','Japan','Japan'],
                   'id_city':[11,21,22,31,32],
                   'city': ['RIYADH','SEOUL','BUSAN','TOKYO','OSAKA'],})

I want to change homeground values in student so the result will be like this

id homeground
1  31
2  2
3  11
4  3
5  31
6  32
7  1
8  21
9  0
10 22

>Solution :

Use:

s1 = student.homeground.map(hg.set_index('city')['id_city'])
s = hg.drop_duplicates(['country']).set_index('country')['id_country'].rename(str.lower)
s2 = student.homeground.str.lower().map(s)

student['homeground'] = s1.fillna(s2).fillna(0, downcast='int')
print (student)
   id  homeground
0   1          31
1   2           2
2   3          11
3   4           3
4   5          31
5   6          32
6   7           1
7   8          21
8   9           0
9  10          22