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Calling AssignHydroID_archydro from a Python script raises a NullReferenceException exception


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Hi, could someone please help me with the following issue: every time I call AssignHydroID_archydro from a Python script I get the following exception:

Traceback (most recent call last):

File "<string>", line 244, in run_nodebug

File "C:\masterImport.py", line 63, in <module>


arcgisscripting.ExecuteError: System.NullReferenceException: Object reference not set to an instance of an object.

at ESRI.APWR.ArcHydroGPTools.AHAssignHydroID.Execute(IArray paramvalues, ITrackCancel trackcancel, IGPEnvironmentManager envMgr, IGPMessages message)

Failed to execute (AssignHydroID).

Here is the relevant part of the Python script:


import arcgisscripting

gp = arcgisscripting.create(9.3)

gp.AddToolbox("C:/Program Files/ArcGIS/ArcToolbox/Toolboxes/Arc Hydro Tools.tbx")

gp.AddToolbox("C:/Program Files/ArcGIS/ArcToolbox/Toolboxes/Arc Hydro Groundwater Tools.tbx")

...various variable definitions

if gp.exists(gwd):

gp.OpenGroundwaterConfig_Groundwater(gwd, gwd_project_name)


gp.CreateGroundwaterConfig_Groundwater(rasters, gwoutput, gwd_project_name, gwd)

if gp.exists(ahd):

gp.OpenArcHydroConfig_archydro(ahd, ahd_project_name)


gp.CreateArcHydroConfig_archydro("Groundwater", rasters, gdb, ahd_project_name, ahd)

fc = r"C:\MMM.gdb\Framework\Well" #the fc lives within the fgdb configured in the .ahd and .gwd files

gp.AssignHydroID_archydro(fc) #this line throws the exception

#gp.AssignHydroID_archydro(fc,"") #these 2 combinations fail too



I am running AHGW 2.0 on ArcGIS 9.3.1 SP1 (ArcInfo license).

The AHGW hardware dongle is plugged in.

Some observations:

-Running the Assign Hydro ID tool from a model works fine

-Trying to call a model, containing the Assign Hydro ID call, from a Python script fails with the same exception as above

-Calling Assign Hydro ID from a Python script on a feature class which has already had all HydroIDs populated works fine

-In summary: AssignHydroID only fails when called from a Python script and it needs to insert new ids.

Could someone please tell me how to get around this problem? I would be very appreciative as calling AssignHydroID_archydro from a script would automate my import process very nicely.

Much obliged,


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This function reproduces part of the AssignHydroID tool's functionality.

It does not take into account individual layers as listed in the LAYERKEYTABLE, as I don't have a need for this.

def assignHydroID(apuniqueid, fc, overwrite=False):
'''apuniqueid: the location of the apuniqueid table
fc: the feature class which needs the HYDROID populated
overwrite: True if the existing HydroIDs in the fc should be overwritten

Populates the HYDROID in the given fc based on the lastest hydroidid from the apuniqueid table

Returns: True if successful
False if the table/fc does not exist

Throws: exception if the table/fc can not be open for editing/inserting

import arcgisscripting
gp = arcgisscripting.create(9.3)

if not gp.exists(apuniqueid):
gp.adderror("APUNIQUEID: %s table does not exist"%apuniqueid)
return False
if not gp.exists(fc):
gp.adderror("%s does not exist"%fc)
return False

lastid = 0
exists = False #True if a HYDROID entry already exists in the apuniqueid table

rows = gp.searchcursor(apuniqueid,"IDNAME = 'HYDROID'")
row = rows.next()
if row:
lastid = row.LASTID
exists = True
del row, rows

rows = gp.updatecursor(fc,"","","HYDROID")
row = rows.next()
while row:
if row.isnull("HYDROID") or overwrite:
row.HYDROID = lastid
row = rows.next()
del row, rows

if exists: #a HYDROID entry already exists, so simply update the field
rows = gp.updatecursor(apuniqueid,"IDNAME = 'HYDROID'")
row = rows.next()
row.LASTID = lastid
del row, rows
else: #need to make a new entry into the table for HYDROID
rows = gp.insertcursor(apuniqueid)
row = rows.newrow()
row.LASTID = str(lastid)
del row, rows

gp.addmessage("Populated the HydroID field in %s."%fc)
gp.addmessage("The next hydroid would be %s"%lastid)
return True

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  • 9 months later...


Please notice that in version 3.0 of the Arc Hydro Groundwater tools there is a new Assign HydroID GW Tool:


You can also call this tool tool from a python script.

With the new tool you don't need the Arc Hydro (surface water) tools or the ApFramework installed as you would in previous versions.


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