python script to calculate field values used to work - now it doesn't.

Question

I have a python script that adds a hectares field and then calculates its
value using the Area field. This worked fine last week but for some
reason it gives null values this week.

code :
# final resultant
gp.Identity_analysis (interimFC, "THLB_Lyr", finalFC )

# Getting the name of the area field for our resultant
descObj = gp.Describe(finalFC)
areaField = descObj.AreaFieldName
print areaField, 'is the fieldname for Area'

# ADD a HECTARES COLUMN = 'AREA_HA'
gp.AddField_management(finalFC, "AREA_HA", "DOUBLE", "", "", "", "",
"NULLABLE", "NON_REQUIRED", "")

# CALCULATE AREA_HA using AREA FIELD
print 'calculating Hectares field'
hectaresCalc = '"' + '[' + areaField + ']/10000' + '"'
print hectaresCalc
gp.CalculateField_management(finalFC, "AREA_HA", hectaresCalc, "VB",
"")
I got that syntax from the ArcToolbox. I don't know why it suddenly
stopped working. I swear this worked last week. Now I get a resultant
dataset that has AREA_HA added but it's values are all null.

Any suggestions or ideas?

KimOllivier · Answer

I've actually tried VB and Python (and every punctuation mark I could think of) and neither works. I ran out of things to try and ended up just doing this in ArcMap. I think the Calculate_Field tool is buggy or something because it works sometimes and othertimes it doesn't.

It is more likely that you have data that causes a crash in the expression. If you use the field calculator you do not have the opportunity to trap data errors in a try/except block and its impossible to see what record caused it.

Since the CalculateField simply wraps the expression in a cursor, it is much much better to use your own cursor in a script and do the expression inside a try/except block. CalculateField is more useful in interactive processes or models.

JoshuaChan · Answer

Seems that you're mixing Python syntax with VB syntax in the CalculateField expression.  VB uses the square bracket notation for field names, as in [my_field] and in Python the notation is exclamation point, as in !my_field!  I noticed that you're using Python in your modelbuilder version but VB in your script.  Try using Python and the exclamation point throughout and see what happens.

I've actually tried VB and Python (and every punctuation mark I could think of) and neither works. I ran out of things to try and ended up just doing this in ArcMap. I think the Calculate_Field tool is buggy or something because it works sometimes and othertimes it doesn't.

DaleHoneycutt · Answer

Seems that you're mixing Python syntax with VB syntax in the CalculateField expression.  VB uses the square bracket notation for field names, as in [my_field] and in Python the notation is exclamation point, as in !my_field!  I noticed that you're using Python in your modelbuilder version but VB in your script.  Try using Python and the exclamation point throughout and see what happens.