Attribute outputField not supported in this instance of FieldMap

Question

Hi. I'm having difficulty with field mapping in arcpy - it keeps throwing this error: "The attribute 'outputField' is not supported on this instance of FieldMap." and I'm not sure why. It never manages to get past the first instance, but if I take that part out it complains that streamorder isn't declared. I swear I copied this almost word for word from the documentation, and I'm really not sure why it's failing.

Not sure if this is the right place for this question, so if there's somewhere else I should ask please let me know!

Here's my code:

#setting workspaces and paths
arcpy.env.workspace = path.join(workspace, outgdb, geodataset)
canadafcs = arcpy.ListFeatureClasses()
print("Geodataset and feature classes found.")

#separating feature classes by whether they abbreviate the field name or not
#creating blank lists
fullwords = []
abbrev = []

#running a for loop to get through the feature classes
for fc in canadafcs:
  fieldlist = arcpy.ListFields(fc)
  if "STREAM_ORDER" in fieldlist:
    fullwords.append(fc)
  elif "STRMRDR" in fieldlist:
    abbrev.append(fc)

print("Feature classes separated by field name format.")

#setting up field mappings:
fm_order = arcpy.FieldMap()
fieldmapping = arcpy.FieldMappings()
fm_order.mergeRule = "First"
print("Field mapping parameters set.")

#if the fc uses full words, add the fc and "STREAM_ORDER" as an input field
for fc in fullwords:
  fm_order.addInputField(fc, "STREAM_ORDER")

#if the fc uses abbreviated words, add the fc and "STRMRDR" as an input field
for fc in abbrev:
  fm_order.addInputField(fc, "STRMRDR")

print("Input fields added.")

#this is where it throws the error - trying to add streamorder as an output field
streamorder = fm_order.outputField
streamorder.name = "Stream_Order"
fm_order.outputField = streamorder

fieldmapping.addFieldMap(fm_order)

NH_MapMan · Answer

Hello,

Replying to this old discussion because I had the same issue and didn't see any good documentation online about it.  I figured out that the "outputField not supported in this instance of FieldMap" error occurs when you try to set the outputField property before you define the input fields using the .addInputField() method.  It seems that you have to define input fields on the FieldMap object before it will allow you to set up the output field properties.  So you need to do

fm.addInputField(layer,field)

before you set the output field.

Another thing to be aware of that tripped me up is that the FieldMap object will not allow you to set properties of the outputField directly.  If you try something like

fm.outputField.name = "NewField"

it will not stick and it will just overwrite whatever properties you give it with the default mapping based on the input fields.  Instead you need to create a copy of the outputField property, update the properties of the copy, and then replace the entire outputField with the copy.  This is shown in the arcpy documentation, but it's a little unclear why it's necessary.

outf = fm.outputField
outf.name = "NewField"
# other properties
fm.outputField = outf

blake.terhune · Answer

Is this any Different in ArcGIS Pro? I used this approach in ArcMap but in transitioning to ArcGIS Pro (2.9), I get an error

Traceback (most recent call last):
  File "C:\Program Files\ArcGIS\Pro\Resources\ArcPy\arcpy\arcobjects\_base.py", line 90, in _get
    return convertArcObjectToPythonObject(getattr(self._arc_object, attr_name))
AttributeError: FieldMap: Get attribute outputField does not exist

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\...", line 223, in execute
    address_eid = fm_tf_eid.outputField
  File "C:\Program Files\ArcGIS\Pro\Resources\ArcPy\arcpy\arcobjects\_base.py", line 96, in _get
    (attr_name, self.__class__.__name__))
AttributeError: The attribute 'outputField' is not supported on this instance of FieldMap.

This is from the geoprocessing window in ArcGIS Pro running my script in a Python toolbox.

SP_WWU · Answer

Thanks Jeff! I ended up not using field mapping after all - I worked around it by renaming the fields in all my different datasets the same thing so they'd map together automatically. Still not sure what went wrong with my program, but it works now.