def season(s):
    if s == 12 or s == 1 or s == 2:
        return 'зима'
    elif s == 3 or s == 4 or s == 5:
        return 'весна'
    elif s == 6 or s == 7 or s == 8:
        return 'лето'
    elif s == 9 or s == 10 or s == 11:
        return  'осень'
    else:
        return 'ошибка'

 def season(month):
    if month in (12, 1, 2):
        return "зима"
    elif month in (3, 4, 5):
        return "весна"
    elif month in (6, 7, 8):
        return "лето"
    elif month in (9, 10, 11):
        return "осень"

 >>>help()

 help>

  
>>> 2 in (1, 2, 3)
True
>>> 4 in (1, 2, 3)
False
>>>

 >>> class IContainEverything:
...      def __contains__(self, what):
...          print("I contain %s too..." % what)
...          return True
>>> storage = IContainEverything()
>>> 1 in storage
I contain 1 too...
True
>>> 'bochka' in storage
I contain bochka too...