Converting an Arcade expression into Python

Question

Currently, I use the following Arcade expression on a field calculation to count the number of attachments a feature has and set that number in the field.

return Count(Attachments($feature))

Is there a simple way to do the same thing in Python? I just want to get a count of the attachments.

JasonBennett · Answer

Thanks for the info. I am trying to update an attribute with the total number of attachments per feature. The data is not static. In my current solution, I already have to periodically run the calculation.

It sounds like Python may not be worth the trouble here, especially since I have a working solution. I want Python to become a common part of my workflow in ArcGIS Pro and thought converting this would be good practice.

JasonBennett · Answer

Thank you for the code snippet. I thought I would have more time to look into this this week but I will take a look at your solution in the future.

bixb0012 · Answer

Not sure if you are using or talking about Field Calculator.  If so, the short answer to your question is, "no."  There is no way using Python with the Field Calculator to emulate the Arcade code for counting attachments.  Outside of Field Calculator, the answer to your question is, "not really."  That is, the ArcPy way of updating a field to reflect the attachment count isn't what most people would consider 'simple,' although it is quite possible.

The following code snippet should work:

import arcpy
import os
import warnings
from collections import defaultdict

table = # path to table
attach_count_field = # field for holding count of attachments

dcn = []
desc = arcpy.Describe(table)
for name in desc.relationshipClassNames:
    rel = arcpy.Describe(os.path.join(desc.path, name))
    if rel.isAttachmentRelationship:
        dcn = rel.destinationClassNames

if not dcn:
    warnings.warn("Table '{}' does not have attachments enabled".format(table))

attach_count = defaultdict(int)
for name in dcn:
    with arcpy.da.SearchCursor(os.path.join(desc.path, name), "REL_OBJECTID") as cur:
        for rel_oid, in cur:
            attach_count[rel_oid] += 1

with arcpy.da.UpdateCursor(table, ["OID@", attach_count_field]) as cur:
    for oid, _ in cur:
        cur.updateRow([oid, attach_count[oid] if dcn else None])

If a table had attachments and they were removed/disabled, the code will set the count field to NULL to reflect attachments are not enabled.