Calculate Field using Python while retaining null values

KatieBudreski — Thu, 22 Apr 2010 17:49:40 GMT

I am trying to use the Calculate Field tool in Model Builder using python (not VB) to convert from meters to miles.

Some records are null, in which case I want them to stay as null.

So, in the expression line (using Python 9.3), if I use the following, then nulls become 0, where in VB these would remain as null:

(!HabitatDistance!) * 0.000621371192

I've also tried to use Calc and Reclass with a variety of if, else statements, but none of those seem to work. I just get 0's for null input values.

e.g.

Expression: Calc(float(!HabitatDistance_mi!),float(!HabitatDistance_m!))

Codeblock:
def Calc(dist1,dist2):
    if dist2 == None:
        return None
    else:
        return dist2 * 0.000621371192

Any solutions?

Calculate Field using Python while retaining null values

DarshaHardy — Fri, 23 Apr 2010 06:56:35 GMT

I had a problem (in 9.2) with nulls and cursors in a python script. Basically, not possible to insert a null. I was getting the value from the row above being copied down instead - absolutely useless. Apparently a known bug, fixed for 9.4/10 and fixed with a service pack for 9.3 - no fix for 9.2 though.

Just found this thread http://forums.esri.com/Thread.asp?c=93&f=1729&t=262236

The comment "Just some advice some might find useful: The code block option in the Calculate Field tool is nothing more than a gp update cursor in disguise" is interesting. Maybe it's the same problem.

topic Calculate Field using Python while retaining null values in Geoprocessing Questions

Calculate Field using Python while retaining null values

Calculate Field using Python while retaining null values