The code

import pandas as pd
def format_city(s):
    '''Input to this function must be of the form 
    "city name, xx" where xx is the state abbreviation.
    Will return a properly formatted version of the
    city and state combination, with the state
    spelled out.'''
    codes = pd.DataFrame({'state':['Alabama','Alaska','Arizona','Arkansas','California','Colorado','Connecticut']},
                      index=['AL','AK','AZ','AR','CA','CO','CT'])
    s = s.split(',')
    abbreviation = s[1].strip().upper()
    return s[0].title() + ', ' + codes.loc[abbreviation]['state']


print(format_city('hartford, ct'))
print(format_city('little rock,    Ar'))

produces the output

Hartford, Connecticut
Little Rock, Arkansas

What is(are) the formal parameter(s) of the function defined in this code?