import pandas

require_list = []
with open('require.csv') as fo:
    for line in fo:
        require_list.append(line.strip())

#Then I see,
id
cat
elephant
dog           # I have elephant in my list.

data = pandas.read_csv("require_database.csv")

  id        kg
0 squirrel  0.1
1 horse     80
2 cat       7
3 bee       0.03
4 dog       12
5 goldfish  0.08      # Unfortunately I do not have data for elephant.

data_list = data[data['id'].isin(require_list)]
print data_list

# I'd like to get something like below....
 id        kg
 cat       7
 elephant  null   # Add "null" behind the elephant..
 dog       12     # Order is the same as my given list.