Using field mapping with ArcPy

Question

I'm having trouble trying to understand how field mapping works in ArcPy. I'm relatively new to Python.

In the below example, I want to take a shapefile and append it to a feature class but I want to use the field map parameter so the fields from the shapefile go to the correct field names in the feature class.

I successfully built this out in ModelBuilder and exported the Python script (see below) but so much of it doesn't make sense.

For example, what are all these different codes... -1, 0, #, true, false, etc?

There seems to be no documentation anywhere online on the meaning behind these codes.

I read that I can use the fieldmapping() class as a parameter in this script but have been unable to incorporate that?

If you know a much easier way (via python) to field map then using the above methods, I'm open to those suggestions.

The code below is only a simple test that I created, so I can hopefully understand field mapping better but in the long run I want to this to be able to handle a LOT of fields from many different shapefiles.

Source shapefile schema: Name, Code, Date

Target feature class schema: Description, Num, Date

# Import arcpy module
import arcpy


# Local variables:
New_Shapefile_shp = "C:\\New_Shapefile.shp"
New_Featureclass__2_ = New_Shapefile_shp
New_Featureclass = "C:\\Target.gdb\\New_Featureclass"

# Process: Append
arcpy.Append_management("C:\\New_Shapefile.shp", New_Featureclass, "NO_TEST", "DESCRIPTION \"DESCRIPTION\" true true false 150 Text 0 0 ,First,#,C:\\New_Shapefile.shp,Name,-1,-1;NUM \"NUM\" true true false 4 Long 0 0 ,First,#,C:\\New_Shapefile.shp,Code,-1,-1;DATE \"DATE\" true true false 8 Date 0 0 ,First,#,C:\\New_Shapefile.shp,Date,-1,-1", "")

Answer 1

Another way of doing it using da.SearchCursor to read shapefile rows and insert them in the fc using da.InsertCursor:

import arcpy

shapefile = r'C:\folder\shapefile.shp'
fc = r'C:\data.gdb\featureclass'

#The position of the fields must match, Name=Description etc.
shapefields = ['Name','Code','Date']
fcfields = ['Description','Num','Date']

icur = arcpy.da.InsertCursor(fc, fcfields+['SHAPE@'])

with arcpy.da.SearchCursor(shapefile, shapefields+['SHAPE@']) as cursor:
    for row in cursor:
        icur.insertRow(row)

del icur

Answer 2

Field mappings are painful, and to be avoided where possible. Therefore @BERA's alternative is a good method, and perhaps better than this one, but for the sake of completeness, here's how I use field mappings...

I have written a Python function that will generate a field mapping object simply by passing it the input feature class (or table) and a dictionary of source-field-names as keys and target-field-names as values. The function returns a field mapping object that can be used in an Append, etc:

def fmapForDict(inputDataset, mappingDict):
    fieldMappings = arcpy.FieldMappings()

    for sourceField in mappingDict:
        fMap = arcpy.FieldMap()
        fMap.addInputField(inputDataset, sourceField)
        outField = fMap.outputField
        outField.name = mappingDict[sourceField]
        fMap.outputField = outField
        fieldMappings.addFieldMap(fMap)

    return fieldMappings

NB: If your source fields and target fields all have the same names, then YOU DO NOT HAVE TO SUPPLY ANY FIELD MAPPINGS AT ALL. The arcpy.Append_management() function with a 'NO_TEST' argument, will automatically map any fields with matching field names (where possible).

PS: The field mappings object returned by this function does not include merge rules, so the default merge rules would apply.