Field Calculator Expression Question (RePost)

[HOWARDWILLIAMS]

Thank you to all who responded to this question earlier.  I am re-posting a more specific question and hopefully my code will be easier to understand.

So, I have number of feature classes in a .gdb that have tree counts per acre in each polygon by year.  I am iteratively unioning the features by year adding a field that is the sum of the previous tree count with the new tree count.

import arcpy
arcpy.CheckOutExtension("spatial")
arcpy.env.overwriteOutput=True
arcpy.env.workspace="C:\LPP\APS.gdb"
R1="R1ADS"
Dam="Damage"
years=['1999','2000', '2001', '2002', '2003', '2004', '2005', '2006', '2007', '2008', '2009', '2010', '2011', '2012', '2013']
fields=['TPA_','Acres_','NoTrees_']
###R1+years(i)+Dam is the name of the feature in the .gdb that will be unioned
RangeYear=['1999_2000','1999_2001','1999_2002','1999_2003','1999_2004','1999_2005',
'1999_2006','1999_2007','1999_2008','1999_2009','1999_2010','1999_2011','1999_2012','1999_2013']
### RangeYear is a variable that I created for naming the unioned outfeature
#________________________________________________

###The statement below create first RangeYear feature to base iteration on in following loop.  Because only two years are being added it requires a simple Field Calculator Expression to sum the trees per acre per year which I was able to figure out.
arcpy.Union_analysis([R1+years[0]+Dam,R1+years[1]+Dam],R1+years[0]+"_"+years[1]+Dam)
for j in range(0,3):
    arcpy.AddField_management(R1+years[0]+"_"+years[1]+Dam,fields+RangeYear[0],"FLOAT","","","")

###I tried to but the statements below in the loop above by having the Field Calculator expressions in a list also, however
###the program would not allow me to iterate through that list so I plugged the expression into the parameters below and ran the tool one at a ###time.
arcpy.CalculateField_management(R1+years[0]+"_"+years[1]+Dam,fields[0]+RangeYear[0],'!TPA_1999!+!TPA_2000!',"PYTHON")
arcpy.CalculateField_management(R1+years[0]+'_'+years[1]+Dam,fields[0]+RangeYear[0],'!Shape_Area! * 0.00024710538',"PYTHON")
arcpy.CalculateField_management(R1+years[0]+'_'+years[1]+Dam,fields[0]+RangeYear[0],'!TPA_1999_2000!*!Acres_1999_2000!',"PYTHON")

###The loop below unions the remaining years the previous years with the next chronological year;
###adds three new field; calculates the new fields values and then move on to the next union.
for i in range(0,13) :
      arcpy.Union_analysis([R1+years[0]+"_"+years[i+1]+Dam,R1+years[i+2]+Dam],R1+years[0]+"_"+years[i+2]+Dam)
             for j in range(0,3):
                  arcpy.AddField_management(R1+years[0]+'_'+years[i+2]+Dam,fields+RangeYear[i+1],"FLOAT","","","")
                  arcpy.CalculateField_management(R1+years[0]+'_'+years[i+2]+Dam,fields, ????Expression????, "PYTHON")
    
###OK so the major crux here is that for TPA_1999_20XX, I need to add the previously unioned TPA_1999_20XX-1 to TPA_20XX.
###I have no idea how to do this.  I suspect that once I figure this out, the next two expressions will be evident.
####################################Thanks for reading, sorry for being such a hack programmer!####################

Field Calculator Expression Arcpy

Message was edited by: HOWARD WILLIAMS

[XanderBakker]

What do you get when you print expression[0]. What is the content of the expression?

In the skeleton that I posted before it generate (I think) a valid syntax for the expression. In see that you are not using all the code that I posted. Maybe you could try the code below or share a part of the data to test if it works and will help to correct any errors.

Please try the code below (on a copy of your data to be sure) and post back any errors it generates:

def main():
    import arcpy
    arcpy.env.overwriteOutput=True
    arcpy.env.workspace= r"C:\LPP\APS.gdb"

    fields = ['TPA_','Acres_','NoTrees_']
    expr = "'!TPA_{0}! + !TPA_{1}!','!Shape_Area! * 0.00024710538','!TPA_{2}! * !Acres_{2}!'"
    years = [str(year) for year in range(1999, 2014)]
    range_years = ['{0}_{1}'.format(years[0], year) for year in years[1:]]

    # initial union
    print "initial union"
    lst_union = ["R1ADS{0}Damage".format(y) for y in years[:2]]
    fc_out = "R1ADS{0}Damage".format(range_years[0])
    print lst_union
    print fc_out
    prev_union = fc_out

    arcpy.Union_analysis(lst_union, fc_out)
    expr2 = expr.format(years[0], years[1], range_years[0])
    expressions = expr2.split(',')
    for j in range(0,3):
        fld = fields+range_years[0]
        print " - AddField", fld
        print " - CalcField", fld, expressions
        arcpy.AddField_management(fc_out, fields+range_years[0], "FLOAT")
        arcpy.CalculateField_management(fc_out, fields+range_years[0], expressions, "PYTHON")

    # other unions
    print "\nother unions"
    for y in years[2:]:
        index = years.index(y)
        lst_union = ["R1ADS{0}Damage".format(years[index-1]), "R1ADS{0}Damage".format(y)]
        lst_union = [prev_union, "R1ADS{0}Damage".format(y)]
        fc_out = "R1ADS{0}Damage".format(range_years[index-1])
        prev_union = fc_out
        print lst_union
        print fc_out
        expr3 = expr.format(range_years[index-2], y, range_years[index-1])
        expressions2 = expr3.split(',')

        arcpy.Union_analysis(lst_union, fc_out)
        for j in range(0,3):
            fld = fields+range_years[index-1]
            print " - AddField", fld
            print " - CalcField", fld, expressions2
            arcpy.AddField_management(fc_out, fld, "FLOAT")
            arcpy.CalculateField_management(fc_out, fields, expressions2, "PYTHON")

if __name__ == '__main__':
    main()