valid_month.py

months = ['January',
          'February',
          'March',
          'April',
          'May',
          'June',
          'July',
          'August',
          'September',
          'October',
          'November',
          'December']

month_abrrs = dict((m[:3].lower(), m) for m in months) # List comprehensions generator 

def valid_month(month):
    if month:
        month = month.strip()[:3].lower()
        if month in month_abrrs:
            return month_abrrs.get(month)
    return None