KafkaAdminClient.create_partitions with int total count fails with InvalidReplicationAssignmentError (sends empty assignments array instead of null)

[ebrard]

Summary

Calling create_partitions({topic: N}) with a plain int total count — the form the docstring recommends — fails against a normal broker with InvalidReplicationAssignmentError (error 39). The int path builds the per-topic struct without an assignments value, which defaults to an empty list [] rather than None. On the wire that becomes a present-but-empty assignments array, which the broker interprets as "manual replica assignment supplied" and then rejects because the assignment count doesn't match the number of new partitions.

Environment

• kafka-python 3.0.9 (latest on PyPI)
• Python 3.14
• Broker: confluentinc/cp-kafka:latest, single node, KRaft mode

Reproduction

from kafka.admin import KafkaAdminClient

admin = KafkaAdminClient(bootstrap_servers="localhost:9092")
# demo-topic already exists with 3 partitions
admin.create_partitions({"demo-topic": 6})

Result:

kafka.errors.InvalidReplicationAssignmentError: [Error 39]
  CreatePartitionsTopic(name='demo-topic', count=6, assignments=[])
  error_message='Attempted to add 3 additional partition(s), but only 0 assignment(s) were specified.'

NewPartitions(total_count=6) is not a usable alternative either — that branch raises TypeError: 'NoneType' object is not iterable because _process_create_partitions_input iterates count.new_assignments unconditionally, even though new_assignments defaults to None.

Root cause (verified at the byte level)

The int path in _process_create_partitions_input builds CreatePartitionsTopic(name=topic, count=count) with no assignments, so it defaults to []. Encoding the request confirms [] and None differ exactly at the assignments field:

# v3 (compact array length byte after count=00000006):
[]   -> ...00000006 01 ...   # compact length 1 => 0 elements, present array
None -> ...00000006 00 ...   # 0 => null

# v0 (classic 4-byte array length):
[]   -> ...00000006 00000000 ...   # length 0, present
None -> ...00000006 ffffffff ...   # -1 => null

The broker treats null as "auto-assign replicas" and a present array as "manual assignment", so the empty array triggers error 39.

Workaround

Build the request so assignments is None:

from kafka.admin._partitions import CreatePartitionsRequest
from kafka.errors import for_code

def create_partitions_auto(admin, topic, total_count):
    Topic = CreatePartitionsRequest.CreatePartitionsTopic
    request = CreatePartitionsRequest(
        topics=[Topic(name=topic, count=total_count, assignments=None)],
        timeout_ms=30000, validate_only=False)
    def errs(r):
        for res in r.results:
            yield for_code(res.error_code)
    return admin._manager.run(admin._send_request_to_controller, request, errs, True)

Suggested fix

• Int path: default assignments to None (not []) when the caller doesn't supply manual assignments.
• NewPartitions path: guard for new_assignments is None before iterating.

[dpkp]

Fix in release 3.0.10