Merge Two layers Keeping only common fields

Question

I have two feature classes that I need to merge to a single layer. There are ~10 common fields that I need to preserve and each of the two layers has some fields that are not shared.

I know that I could first DeleteField_management() each layer to have layers with identical fields and then do a simple merge but it seems like I should be able to use Merge_management() and a field mapping. I don't see how to include/exclude fields though, only how to transform them.

Inputs: file_1 with fields a,b,c,d. file_2 with fields c,d,e,f.

Desired output: file_3 with fields c,d.

Answer 1

Creating layers with hidden fields is the way to go here, using field info objects. I have a function I regularly use for this. Then it's a matter of creating your two layers and using them as inputs in your merge.

Try:

import arcpy
fc1 = r"your\first\featureclass"
fc2 = r"your\second\featureclass"
mergeFc = r"your\output\merge\featureclass"

##function for creating layer with limited fields
def CreateFieldInfoLayer (fc, outLayer, flds, sql = None):
    """
    fc = input feature class
    outLayer = name of output layer
    flds = fields that will be visible in new layer
    sql = option where clause to apply to layer
    """
    #create layer with an unused name
    layer = "templayer"
    arcpy.MakeFeatureLayer_management (fc, layer)
    #get field info
    fldInfo = arcpy.Describe (layer).fieldInfo
    #set fields no in input list to hidden
    for i in range (fldInfo.count):
            fldName = fldInfo.getFieldName (i)
            if not fldName in flds:
                    fldInfo.setVisible (i, "HIDDEN")
    #create new layer with field info applied
    arcpy.MakeFeatureLayer_management (layer, outLayer, sql, None, fldInfo)
    #delete initial layer
    arcpy.Delete_management (layer)
    return outLayer

#get common fields
flds = [f.name for f in arcpy.ListFields (fc1)]
flds = [f.name for f in arcpy.ListFields (fc2) if f.name in flds]

#create layers with unshared fields hidden
lyr1 = CreateFieldInfoLayer (fc1, "lyr1", flds)
lyr2 = CreateFieldInfoLayer (fc2, "lyr2", flds)

#merge
arcpy.Merge_management(([lyr1, lyr2], mergeFc)

Answer 2

This is potentially less performant than Emil's answer, but much more concise: just merge everything indiscriminately, create 2 sets with the original field names, then delete the symmetric difference of those field names (belonging to either fc_a or fc_b, but not both).

merged = arcpy.Merge_management([fc_a, fc_b], 'merged')
fields_a = {field.name for field in arcpy.ListFields(fc_a)}
fields_b = {field.name for field in arcpy.ListFields(fc_b)}
arcpy.DeleteField_management(merged, list(fields_a ^ fields_b))

Answer 3

so I just wanted to add the solution i ended up going with, a hybrid of a few different ones.

It's frustrating the the esri documentation for Merge_Management() doesn't actually say how it keeps/deletes fields but it will.

relevant code sample below.

pointfieldmappings = arcpy.FieldMappings()

point_keep_fields = ['RequestID','Prioritization_Bucket','Date_Initiated','Match_addr','X','Y',
                       'Description','Year','full_address','Sidewalk_Presence_Criteria',
                       'Bike_Network_Criteria','Lighting_Presence_Criteria','Equity_Criteria',
                       'City_Plans_Criteria','Crash_History_Criteria','Community_Requests_Criteria',
                       'Transit_Criteria','Functional_Classification_Criteria','Community_Center_Services_Criteria',
                       'Commercial_Retail_Criteria','Proximity_to_existing_speed_humps_Criteria','Senior_Center_Housing_Criteria','School_Zone_Criteria','Prioritization_Score','Priority_Bucket']
        
pointfieldmappings.addTable(point_table)
pointfieldmappings.addTable(point_master)
for field in pointfieldmappings.fields:
    if field.name not in point_keep_fields and not field.required:
                    pointfieldmappings.removeFieldMap(pointfieldmappings.findFieldMapIndex(field.name))
    
point_combined_results = point_table + "_all"
                        
arcpy.Merge_management(inputs = [point_table, point_master],
                                   output=point_combined_results,
                                   field_mappings=pointfieldmappings)

the point_keep_fields variable could also be built using the a_fields ^ b_fields code, but in my case that wasn't exactly what I needed.