Converting Field Calculator Pre-Logic Script to pyscript

[CliveCartwright]

I've been struggling to emulate a simple Field Calculator function as pyscript. When I try, I get parsing errors.

For example, how should this simple function (when manually applied in the Field Calculator tool) look when written up as pyscript?

In Pre-Logic Script Code:

def LookUp ( a, b ) :

  if ( a == 0 ) : 

    newval = 0

  else :

    newval = b

  return newval

In FIELD =

LookUp ( !MAX_RANK!, !Shape_Area! )

Any advice will be much appreciated.

[DanPatterson_Retired]

Try this... if there are any errors it is because I can't test it from an iThingy since you can't run Arc* stuff on them

You basically want an update cursor

UpdateCursor—Help | ArcGIS for Desktop

import arcpy
fc = "c:/path_to/Your_gdb/my.gdb/yourlayer"
field1 = "Shape_Area"
field2 = "MAX_RANK"
calc_fld = "Calculate_field"
cursor = arcpy.UpdateCursor(fc)
row = cursor.next()
while row:
    # field2 will be equal to field1 multiplied by 3.0
    if row.getValue(field1) == 0.0:
        row.setValue(0.0)
    else:
        row.setValue(calc_fld, row.getValue(field1))
    cursor.updateRow(row)
    row = cursor.next()

[CliveCartwright]

Thanks Dan.

I've just tried this following your advice and sadly it returned a runtime error:

fc = currentWorkspace + "/dataset"

field1 = "MAX_RANK"

field2 = "Shape_Area"

calc_fld = "Calculate_field"

cursor = arcpy.UpdateCursor(fc)

row = cursor.next()

while row:

    if row.getValue(field1) == 0:

        row.setValue(0)

    else:

        row.setValue(calc_fld, row.getValue(field2))

    cursor.updateRow(row)

    row = cursor.next()

Runtime error

Traceback (most recent call last):

  File "<string>", line 35, in <module>

  File "c:\program files (x86)\arcgis\desktop10.1\arcpy\arcpy\arcobjects\arcobjects.py", line 1040, in setValue

    return convertArcObjectToPythonObject(self._arc_object.SetValue(*gp_fixargs(args)))

AttributeError: Row: Error in parsing arguments for SetValue

I'll keep chipping away at it.

[WesMiller]

The simplest way to get the proper syntax is to use your calculation in model builder then export as python script.

[DanPatterson_Retired]

Wes... which assumes the model works first go... but based on the number of model failures on this site, that would be my last choice

[CliveCartwright]

Thanks Wes. I tried this and it returned:

Failed to execute. Parameters are not valid.

ERROR 000989: Python syntax error: Parsing error SyntaxError: invalid syntax (line 1)

Failed to execute (CalculateField).

[WesMiller]

Could you post a screen shot of the model builder field calculator.

[CliveCartwright · Accepted solution]

I have made it work!!!!!!

Code I used:

# Populate SUBJECT_AREA only if there is a Rank shown in MAX_RANK field.

expression = "LookUp(!MAX_RANK!, !Shape_Area!)"

codeblock = "def LookUp(MAX_RANK, Shape_Area):\n\

    if MAX_RANK == 0:\n\

        return 0\n\

    else:\n\

        return Shape_Area"

gp.CalculateField_management(currentWorkspace + "/myDataset", "SUBJECT_AREA", expression, "PYTHON", codeblock)