Python Elasticsearch update with script exception

I am trying to use python ES client to append the new object into the array of document (see code snippet below). But I got the RequestError(400, 'illegal_argument_exception', 'failed to execute script')

Python code:

def index_doc(doc_id, index):
    try:
        body = {
            "script": {
                "source": "ctx._source.array_1.add(params.data)",
                "lang": "painless",
                "params": {
                    "data": {
                        "field1": "abc",
                        "field2": 3
                    }
                }
            }
        }

        client.update(
            index=index,
            id=doc_id,
            routing=doc_id,
            body=body
        )
    except Exception as e:
        print("exception - ", doc_id, e)

Tried "script" parameter too, got same RequestError(400, 'illegal_argument_exception', 'failed to execute script'):

def index_doc(doc_id, index):
    try:
        script = {
            "source": "ctx._source.array_1.add(params.data)",
            "lang": "painless",
            "params": {
                "data": {
                    "field1": "abc",
                    "field2": 3
                }
            }
        }

        client.update(
            index=index,
            id=doc_id,
            routing=doc_id,
            script=script
        )
    except Exception as e:
        print("exception - ", doc_id, e)

I tested the same update payload with postman command, there is no any problem, the new object can be inserted into the array of document. See curl command:

curl --location --request POST 'https://<endpoint>/<index_name>/_update/<doc_id>?routing=<doc_id>' \
--header 'Content-Type: application/json' \
--header 'Authorization: Basic <token>' \
--data-raw '{
    "script": {
        "source": "ctx._source.array_1.add(params.data)",
        "lang": "painless",
        "params": {
            "data": {
                "field1": "abc",
                "field2": 3
            }
        }
    }
}'

So, could anyone share thoughts what might be the culprit? I am using ES 7.17. Thanks much in advance.

Update the trackback log here:

  client.update(
exception -  <doc_id> RequestError(400, 'illegal_argument_exception', 'failed to execute script')
Traceback (most recent call last):
  File "/<file_path>", line 137, in index_doc
    client.update(
  File "/Library/Python/3.9/site-packages/elasticsearch/client/utils.py", line 347, in _wrapped
    return func(*args, params=params, headers=headers, **kwargs)
  File "/Library/Python/3.9/site-packages/elasticsearch/client/__init__.py", line 2092, in update
    return self.transport.perform_request(
  File "/Library/Python/3.9/site-packages/elasticsearch/transport.py", line 466, in perform_request
    raise e
  File "/Library/Python/3.9/site-packages/elasticsearch/transport.py", line 427, in perform_request
    status, headers_response, data = connection.perform_request(
  File "/Library/Python/3.9/site-packages/elasticsearch/connection/http_urllib3.py", line 291, in perform_request
    self._raise_error(response.status, raw_data)
  File "/Library/Python/3.9/site-packages/elasticsearch/connection/base.py", line 328, in _raise_error
    raise HTTP_EXCEPTIONS.get(status_code, TransportError)(
elasticsearch.exceptions.RequestError: RequestError(400, 'illegal_argument_exception', 'failed to execute script')

Per the official documentation (8.x), you don't specify a body but the script directly:

        client.update(
            index=index,
            id=doc_id,
            routing=doc_id,
            script=script            <------
        )

Figured out the issue.
The array in the doc does not exists at the beginning, so if I directly add object into the array, it will have the error mentioned in the question. When I tried the curl command yesterday, the doc id is different, so the curl didn't trigger the error. So, I added the null checks like below, then the issue is resolved.

Also, thanks @Val for sharing his knowledge.

"source" : "if (ctx._source.array_1 == null) { ctx._source.array_1 = [params.data]; } else {ctx._source.array_1.add(params.data)} "