I would probably do this as follows:
import datetime, os
current_month = datetime.date.today().replace(day=1)
# this could be more concise if `start_month` were the month number rather than month name
possible_month = datetime.datetime.strptime('%s %s' % (start_month, start_year), '%B %Y').date()
while possible_month <= current_month:
csv_filename = possible_month.strftime('%B %Y') + '.csv'
if os.path.exists(csv_filename):
CSVFile = True
# do something cool here, and maybe break the loop if you like
possible_month = (possible_month + datetime.timedelta(days=31)).replace(day=1)
Let me know if you need me to expand on how that works.