Validate and calculate fields

validate_and_calculate_field.py

def validate_and_calculate_field(layer, field, expression):

    # Validate field exists
    fields = [f.name for f in arcpy.ListFields(layer)]
    if field not in fields:
        raise ValueError(f"Field {field} does not exist in layer")

    # Validate expression fields exist
    def get_fields_from_expression(expression):
        matches = re.findall(r"!\w+!", expression)
        return [m[1:-1] for m in matches]

    expr_fields = get_fields_from_expression(expression)
    missing_fields = set(expr_fields) - set(fields)
    if missing_fields:
        raise ValueError(f"Expression contains invalid fields: {missing_fields}")

    # Calculate field if valid
    arcpy.management.CalculateField(layer, field, expression, "PYTHON3")

# usage
validate_and_calculate_field("somelayer", "TARGETFIELD", "!SOME_FIELD!.upper()+'_'+!SOME_OTHER_FIELD!")