day in year | Python Fiddle

# A python function that takes in three integer arguments (year, month, day) and returns an integer from 1 to 366 representing
# the day within the year.  For example, day_in_year(12,1,1) returns 1, day_in_year(2012,2,29) returns 60, and 
# day_in_year(2012,12,31) returns 366.

def is_leap_year(year):
    # years divisible by 400 (such as 1600 or 2000) are exceptions to the usual "divisible by 4" rule
    if year%400 == 0:
        return False
    else:
        # years divisible by 4 are leap years, unless they are also divisible by 400
        if year%4 == 0:
            return True
        else:
            return False

def day_in_year(year, month, day):
    # Concept:
    # For Jan 15, the return value should simply be 15.
    # For Feb 15, the return value should be however many days were in Jan (the prior month), plus 15.
    # For March 15, the return value should be however many total cumulateive days were in Jan and Feb, (the prior months) plus 15.
    
    # Examples in the code:
    # For inputs of of a February 15 date (month=2),  we must look up a value of 31 (the "january prior month cumulative total") and then add 15.
    # To get a total value of 31 for the "first" month of Jan, we need a list with 31 in the "first" position, but lists are indexed starting with zero.
    # Since month=2 for Feb input, we have to subtract 2 from the input to get zero, and retrieve 31 as a "total days in the prior months"
    # For an input of March 15 (month=3)  3-2=1, so prior_months_days[1] will produce 59, the total days in Jan and Feb that must be added to 15
    # However, if it's a leap year, we need to add 1 to that total, so that's why we have a leap_year_correction.
    # For inputs of January (month=1), when we subract 2 from month, we get -1, and a list lookup of listname[-1] produces the last item in the list.
    # There's no 365 in the list because the highest month that should come in the input is 12, and 12-2=10, so prior_months_days[10] is 334, and that's
    # sufficient
    
    leap_year_correction = 1 if month > 2 and is_leap_year(year) else 0
    prior_months_days = [31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 0]
    return prior_months_days[month - 2] + day + leap_year_correction

def test():
    test_cases = [((2012, 1,   1),   1), 
                  ((2012, 1,  31),  31),
                  ((2012, 2,   1),  32),
                  ((2012, 2,  29),  60),
                  ((2012, 3,   1),  61),
                  ((2012, 3,  31),  91),
                  ((2012, 4,   1),  92),
                  ((2012, 4,  30), 121),
                  ((2012, 5,   1), 122),
                  ((2012, 5,  31), 152),
                  ((2012, 6,   1), 153),
                  ((2012, 6,  30), 182),
                  ((2012, 7,   1), 183),
                  ((2012, 7,  31), 213),
                  ((2012, 8,   1), 214),
                  ((2012, 8,  31), 244),
                  ((2012, 9,   1), 245),
                  ((2012, 9,  30), 274),
                  ((2012, 10,  1), 275),
                  ((2012, 10, 31), 305),
                  ((2012, 11,  1), 306),
                  ((2012, 11, 30), 335),
                  ((2012, 12,  1), 336),
                  ((2012, 12, 31), 366),
                  ((2013, 1,   1),   1),
                  ((2013, 1,  31),  31),
                  ((2013, 2,   1),  32),
                  ((2013, 2,  28),  59),
                  ((2013, 3,   1),  60),
                  ((2013, 3,  31),  90),
                  ((2013, 4,   1),  91),
                  ((2013, 4,  30), 120),
                  ((2013, 5,   1), 121),
                  ((2013, 5,  31), 151),
                  ((2013, 6,   1), 152),
                  ((2013, 6,  30), 181),
                  ((2013, 7,   1), 182),
                  ((2013, 7,  31), 212),
                  ((2013, 8,   1), 213),
                  ((2013, 8,  31), 243),
                  ((2013, 9,   1), 244),
                  ((2013, 9,  30), 273),
                  ((2013, 10,  1), 274),
                  ((2013, 10, 31), 304),
                  ((2013, 11,  1), 305),
                  ((2013, 11, 30), 334),
                  ((2013, 12,  1), 335),
                  ((2013, 12, 31), 365)]
    passes = 0
    for (args, answer) in test_cases:
        result = day_in_year(*args)
        if result != answer:
            print "Failed when argument was: ", args
        else:
            passes = passes + 1
    print str(passes) + " out of " + str(len(test_cases)) + " test cases passed."

test()
#print day_in_year(2012, 2, 1)

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