You can call operator.itemgetter() as a sort_key value. Note that sortby still needs to be given for the sort_key to be applied:

import operator
from prettytable import PrettyTable


table = PrettyTable(["Name", "Grade"])
table.add_row(["Joe", 90])
table.add_row(["Sally", 100])
table.add_row(["Bill", 90])
table.add_row(["Alice", 90])
print table.get_string(sort_key=operator.itemgetter(1, 0), sortby="Grade")

Prints:

+-------+-------+
|  Name | Grade |
+-------+-------+
| Alice |   90  |
|  Bill |   90  |
|  Joe  |   90  |
| Sally |  100  |
+-------+-------+

You can do it with sort_key argument where custom key function is passed to:

#!/usr/bin/python3

from prettytable import from_csv

with open("data.csv", "r") as fp: 
    x = from_csv(fp)

print(x.get_string(sortby='Population', sort_key=lambda row: int(row[0])))

Output:

+-----------+----------+------------+-----------------+
| City name |   Area   | Population | Annual Rainfall |
+-----------+----------+------------+-----------------+
|   Darwin  | 11200000 |   120900   |      1714.7     |
|   Hobart  |   1357   |   205556   |      619.5      |
|  Adelaide |   1295   |  1158259   |      600.5      |
|   Perth   |   5386   |  1554769   |      869.4      |
|  Brisbane |   5905   |  1857594   |      1146.4     |
| Melbourne |   1566   |  3806092   |      646.9      |
|   Sydney  |   2058   |  4336374   |      1214.8     |
+-----------+----------+------------+-----------------+