Calculate number of Sundays that fell on the first of the month during the twentieth century.

no_sundays.py

def check_if_leap_year(year: int) -> bool:
	if (
		year % 4 == 0 and year % 100 != 0
	) or (
		year % 400 == 0
	):
		return True
	else:
		return False


def get_days_in_month(month: int, year: int) -> int:
	no_days_in_month = [
		31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
	]
	days_to_increment = 0
	if check_if_leap_year(year) and month == 2:
		days_to_increment = 1
	return no_days_in_month[month - 1] + days_to_increment

# track days of week as an integer
# 0 Sun
# 1 Mon
# 2 Tue
# 3 Wed
# 4 Thu
# 5 Fri
# 6 Sat

# starting day: 1 Jan 1901 was a tuesday
day_of_week = 2
count_sunday = 0
for year in range(1901, 2000 + 1):
	for month in range(1, 12 + 1):
		days_in_month = get_days_in_month(month, year) + 1 # + 1 for calculating start of next month
		# calculate day of week for start of every month
		day_of_week = (day_of_week + days_in_month % 7 + 7 * int(days_in_month % 7 == 0) - 1) % 7
		if day_of_week == 0:  # if sunday
			count_sunday += 1
print(count_sunday)