Can take xcom value from the dag, but not within PythonOperator - airflow

Question

I am trying to pull an xcom from within a PythonOperator, but I think I might be missing an element of understanding.
This is my dag :
# dag arguments
default_args = {
"start_date": datetime(2022, 11, 23),
"retries": 1,
"retry_delay": timedelta(minutes=5),
# "max_active_runs": 3,
"schedule_interval": "#daily",
"catchup": False, # enable if you don't want historical dag runs to run
# "dagrun_timeout": timedelta(minutes=20),
}
with DAG(
dag_id="migrate_dtm",
default_args=default_args,
) as dag:
load_data_migration_mapping = PythonOperator(
task_id="load_data_migration_mapping",
python_callable=load_data_migration_map,
)
list_objects = GCSListObjectsOperator(
task_id="list_objects",
bucket=LEGACY_DTM_HISTORY_BUCKET,
prefix="raw_data/datamart/{{ds_nodash}}",
impersonation_chain=IMPERSONATED_SERVICE_ACCOUNT, # Only use in dev env
)
match_mapping_with_data = PythonOperator(
task_id="match_mapping_with_data",
python_callable=match_data_with_migration_map,
provide_context=True,
op_kwargs={
"initial_migration_map": "initial_migration_map",
# "files_and_prefixes": "{{task_instance.xcom_pull(task_ids='list_objects')}}",
"files_and_prefixes": "list_objects",
},
)
(load_data_migration_mapping >> list_objects >> match_mapping_with_data)
In my third PythonOperator : match_mapping_with_data, when I pass the following macro
"files_and_prefixes": "{{task_instance.xcom_pull(task_ids='list_objects')}}"
I can see that it is correctly interpreted within the logs. But when I try to only give it the list_objects task name, in order to perform an xcom_pull in the subsequent function match_data_with_migration_map I get a None.
A partial content of my function until the point where it breaks :
def match_data_with_migration_map(
ti,
**kwargs,
):
# Set arguments' (default) values
xcom_initial_migration_map = kwargs.setdefault(
"initial_migration_map",
"initial_migration_map",
)
xcom_files_and_prefixes = kwargs.setdefault(
"files_and_prefixes",
"list_objects",
)
# Parse Xcoms:
initial_migration_map = ti.xcom_pull(
key=xcom_initial_migration_map,
)
if not initial_migration_map:
err_msg = (
"The xcom initial_migration_map has not "
f"been correctly set : {initial_migration_map}"
)
raise XcomNULL(err_msg)
print(
f"xcom : initial_migration_map - Key: {xcom_initial_migration_map}, "
f"Value = {initial_migration_map}"
)
files_and_prefixes = ti.xcom_pull(
key=xcom_files_and_prefixes,
)
if not files_and_prefixes:
err_msg = (
"The xcom files_and_prefixes has not "
f"been correctly set : {files_and_prefixes}"
)
raise XcomNULL(err_msg)
print(
f"xcom : files_and_prefixes - Key: {xcom_files_and_prefixes}, "
f"Value = {files_and_prefixes}"
)
The custom class..
class XcomNULL(Exception):
pass
N.B : The initial_migration_map XCom is correctly pulled because I set it in the first PythonOperator: load_data_migration_mapping via ti.xcom_push(key="initial_migration_map")
EDIT (Kind of a workaround) :
It works when I set : "files_and_prefixes": "return_value",, how do I rename this XCom in order to have several within my dag run?