python，求解程序中的问题

month=[
          'January',
          'February',
          'March',
          'April',
          'May',
          'June',
          'July',
          'August',
          'September',
          'October',
          'November',
          'December'
]
ending=['st','nd','rd']+17*['th']\
        +['st','nd','rd']+7*['th']\
        +['st']
year=raw_input('Year:')
month=raw_input('Month(1-12):')
day=raw_input('Day(1-31):')
month_number=int(month)
day_number=int(day)
month_name=months[month_number-1]
ordinal=day+ending[day_number-1]
print month_name+' '+ordinal+', '+year



请教month_name=months[month_number-1]这句代码哪里有问题
shell中显示   month_name=months[month_number-1]
NameError: name 'months' is not defined