load.py

from google.cloud import bigquery
c = bigquery.Client()
job_config = bigquery.LoadJobConfig(
    source_format=bigquery.SourceFormat.NEWLINE_DELIMITED_JSON,
    write_disposition='WRITE_APPEND')
job = c.load_table_from_json([{'id': '7', 'value': 'qux'}], 'test_dataset.test_table', job_config=job_config)
print(job.result())

repro.sh

bq mk test_dataset.test_table id:string,value:string
bq load --source_format=NEWLINE_DELIMITED_JSON test_dataset.test_table test.jsonl
python3 load.py

test.jsonl

{"id": "5", "value": "foo"}
{"id": "6", "value": "bar"}