Day of the year calculation in ArcPy syntax help

[RyanHowell1]

I am trying to calculate a "day of the year" field in a feature class using ArcPy. 

###create day update cursor
cursorD = arcpy.da.UpdateCursor(featureclass,["month", "day", "dayoftheyear"])
print("day cursor set")

###loop and assign day of the year
for updateRowD in cursorD:
    if 'month' == 1:
        print("Jan")
        "dayoftheyear" = [day]
        cursorD.updateRow(updateRowD)
        print("Jan completed")
    elif 'month' == 2:
        print("Feb")
        "dayoftheyear" = [day] + 31
        cursorD.updateRow(updateRowD)
        print("Feb completed")
    elif 'month' == 3:
        print("Mar")
        "dayoftheyear" = [day] + 59
        cursorD.updateRow(updateRowD)
        print("Mar completed")‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

and continues throughout the rest of the year (April through December). I'm getting syntax errors on the lines where I actually calculate the day number (lines 9, 14, 19, etc). I just can't figure out the syntax to get it to read the value of a field (day), perform basic arithmetic on it, and assign it to another field (dayoftheyear). I've used lots of combinations of [day], "day", !day!, etc and haven't been able to find my answer in any documentation.

Thanks in advance for your time and suggestions.

[StephenM]

I think the proper syntax is to access the fields using their indexes, as ordered in your fields list. For example:

cursorD = arcpy.da.UpdateCursor(featureclass,["month", "day", "dayoftheyear"])

for updateRowD in cursorD:
	if updateRowD[0] == 1:
		updateRowD[2] = updateRowD[1]
	elif updateRowD[0] == 2:
		updateRowD[2] = updateRowD[1] + 31
	elif updateRowD[0] == 3:
		updateRowD[2] = updateRowD[1] + 59

	cursorD.updateRow(updateRowD)

del updateRowD
del cursorD‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

The del statements are used to clear potential locks on the targeted feature class. You might consider creating the cursor using a with statement so that locks are cleared automatically:

with arcpy.da.UpdateCursor(featureclass,["month", "day", "dayoftheyear"]) as cursorD:‍
	for updateRowD in cursorD:‍‍‍‍‍‍
		if updateRowD[0] == 1:
			updateRowD[2] = updateRowD[1]
		elif updateRowD[0] == 2:
			updateRowD[2] = updateRowD[1] + 31
		elif updateRowD[0] == 3:
			updateRowD[2] = updateRowD[1] + 59

		cursorD.updateRow(updateRowD)‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

You may also want to revise your print statements, since they'll print at every row.

[JoshuaBixby]

I suggest you read Accessing data using cursors—Help | ArcGIS Desktop  , it will help you understand how to work with cursors.  Second, are you three fields just numeric/integer?

[DarrenWiens2 · Accepted solution]

Save yourself the heartache of dealing with dates semi-manually by relying on the datetime module, which will handle it in one line and deal with leap years:

from datetime import datetime

year = 2020
month = 2
day = 29

day_of_year = datetime(year, month, day).timetuple().tm_yday
print(day_of_year)

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