Adding fields to : TypeError: Object of type PropertyMap is not JSON serializable

geo_sunriver — Thu, 25 Sep 2025 17:44:35 GMT

Hi Everyone,

I'm following the ArcGIS API for Python docs in attempts to add a long list of fields to a Table.

The code example I'm trying to replicate from the docs is here below:

-----------------------------------------------------
>>> new_field = {
"fields": [
{
"name": "Loc Identifier",
"type": "esriFieldTypeString",
"alias": "safa",
"nullable": True,
"editable": True,
"length": 256,
}
]
}
>>> res = fl.manager.add_to_definition(
json_dict=add_field
)
>>> res

-----------------------------------------------------

My object of new fields which prints type of <class 'dict'> is shown here below:

-----------------------------------------------------

fields_to_add = []
for field in source_fields:
# Check if the field should be ignored or if it already exists
if field['name'].lower() not in fields_to_ignore_lower and field['name'].lower() not in target_field_names:
fields_to_add.append(field)

field_dict = {"fields":fields_to_add}
print(field_dict)

{'fields': [{
  "name": "PropID",
  "type": "esriFieldTypeInteger",
  "alias": "PropID",
  "sqlType": "sqlTypeOther",
  "nullable": true,
  "editable": true,
  "domain": null,
  "defaultValue": null
}, {
  "name": "SRLaneLot",
  "type": "esriFieldTypeString",
  "alias": "SRLaneLot",
  "sqlType": "sqlTypeOther",
  "length": 255,
  "nullable": true,
  "editable": true,
  "domain": null,
  "defaultValue": null
}, { ... continues on with the rest of the fields ...

When I try to use the 'add_to_definition' method I get the following type error:

result = target_table.manager.add_to_definition(field_dict)
print(result)

TypeError: Object of type PropertyMap is not JSON serializable

As I type this out I realise the docs are showing the add_to_definition method for a type of FeatureLayer. Does anyone know how to add fields to a table that exists as a relationship class in ArcGIS Online?

Re: Adding fields to : TypeError: Object of type PropertyMap is not JSON serializable

geo_sunriver — Thu, 25 Sep 2025 18:28:05 GMT

The solution was right on front of me...

field_dict_list = []
for f in fields_to_add:
field_dict_list.append(dict(f))

I just needed to convert the type of PropertyMap to a dict, append to a list. Then use the list of dictionaries to update the table.

topic Re: Adding fields to <class 'arcgis.features.layer.Table'>: TypeError: Object of type PropertyMap is not JSON serializable in ArcGIS API for Python Questions

Adding fields to : TypeError: Object of type PropertyMap is not JSON serializable

Re: Adding fields to : TypeError: Object of type PropertyMap is not JSON serializable