TypeError: isinstance when azure-identity is not imported using Azure Service Bus transport

[jesusgarmanz]

I'm currently in process of upgrading these three packages:
kombu 5.1.0 -> 5.3.5
celery 5.1.2 -> 5.3.6
azure-servicebus 7.8.1 -> 7.11.4

But found that Celery was throwing a TypeError when it starts up:

  -------------- celery@620caf9bceb7 v5.3.6 (emerald-rush)
 --- ***** -----
 -- ******* ---- Linux-6.6.12-linuxkit-x86_64-with 2024-02-26 15: 36: 17
 - *** --- * ---
 - ** ---------- [config]
 - ** ---------- .> app:         my_app: 0x7ffffdc97370
 - ** ---------- .> transport:   azureservicebus://RootManageSharedAccessKey:my_key@my_env
 - ** ---------- .> results:     redis://redis:6379/
 - *** --- * --- .> concurrency: 8 (prefork)
 -- ******* ---- .> task events: OFF (enable -E to monitor tasks in this worker)
 --- ***** -----
  -------------- [queues]
                 ...


 [tasks]
   ...

 Traceback (most recent call last):
   File "/root/.local/lib/python3.10/site-packages/celery/worker/worker.py", line 202, in start
     self.blueprint.start(self)
   File "/root/.local/lib/python3.10/site-packages/celery/bootsteps.py", line 116, in start
     step.start(parent)
   File "/root/.local/lib/python3.10/site-packages/celery/bootsteps.py", line 365, in start
     return self.obj.start()
   File "/root/.local/lib/python3.10/site-packages/celery/worker/consumer/consumer.py", line 340, in start
     blueprint.start(self)
   File "/root/.local/lib/python3.10/site-packages/celery/bootsteps.py", line 116, in start
     step.start(parent)
   File "/root/.local/lib/python3.10/site-packages/celery/worker/consumer/connection.py", line 21, in start
     c.connection = c.connect()
   File "/root/.local/lib/python3.10/site-packages/celery/worker/consumer/consumer.py", line 469, in connect
     conn = self.connection_for_read(heartbeat=self.amqheartbeat)
   File "/root/.local/lib/python3.10/site-packages/celery/worker/consumer/consumer.py", line 475, in connection_for_read
     return self.ensure_connected(
   File "/root/.local/lib/python3.10/site-packages/celery/worker/consumer/consumer.py", line 526, in ensure_connected
     conn = conn.ensure_connection(
   File "/root/.local/lib/python3.10/site-packages/kombu/connection.py", line 406, in ensure_connection
     self._ensure_connection(*args, **kwargs)
   File "/root/.local/lib/python3.10/site-packages/kombu/connection.py", line 459, in _ensure_connection
     return retry_over_time(
   File "/root/.local/lib/python3.10/site-packages/kombu/utils/functional.py", line 318, in retry_over_time
     return fun(*args, **kwargs)
   File "/root/.local/lib/python3.10/site-packages/kombu/connection.py", line 934, in _connection_factory
     self._connection = self._establish_connection()
   File "/root/.local/lib/python3.10/site-packages/kombu/connection.py", line 860, in _establish_connection
     conn = self.transport.establish_connection()
   File "/root/.local/lib/python3.10/site-packages/kombu/transport/virtual/base.py", line 975, in establish_connection
     self._avail_channels.append(self.create_channel(self))
   File "/root/.local/lib/python3.10/site-packages/kombu/transport/virtual/base.py", line 953, in create_channel
     channel = self.Channel(connection)
   File "/root/.local/lib/python3.10/site-packages/kombu/transport/azureservicebus.py", line 136, in __init__
     self._try_parse_connection_string()
   File "/root/.local/lib/python3.10/site-packages/kombu/transport/azureservicebus.py", line 144, in _try_parse_connection_string
     if (isinstance(self._credential, DefaultAzureCredential) or
 TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

So, digging a little bit on azureservicebus.py file, it tries to import DefaultAzureCredential and ManagedIdentityCredential. If azure-identity package is not installed, kombu sets DefaultAzureCredential and ManagedIdentityCredential as vars with None value.

[...]
# line 72 to 77
try:
    from azure.identity import (DefaultAzureCredential,
                                ManagedIdentityCredential)
except ImportError:
    DefaultAzureCredential = None
    ManagedIdentityCredential = None
[...]

Then, on _try_parse_connection_string function after parsing the URI and getting the credential, kombu checks if the credential is an instance of DefaultAzureCredential and ManagedIdentityCredential.

[...]
# line 144 to 146
if (isinstance(self._credential, DefaultAzureCredential) or
           isinstance(self._credential, ManagedIdentityCredential)):
    return None
[...]

The issue here is when DefaultAzureCredential and ManagedIdentityCredential are set to None, Python can't check if self._credential is an instance of None and throws a TypeError. I don't know if this is intended or not, but I did some changes on my computer and found a easy way to test it maintaining the same import strategy:

# I set the conditions on variables to have more clarity about what is being checked
default_azure_credential = (DefaultAzureCredential is not None and
    isinstance(self._credential, DefaultAzureCredential))
managed_identity_credential = (ManagedIdentityCredential is not None and
    isinstance(self._credential, ManagedIdentityCredential))
if default_azure_credential or managed_identity_credential:
    return None

I could open a PR if this is some unexpected behavior.

[jesusgarmanz]

Clarify that I am using the same configuration on Celery, Kombu and Service Bus before and after the upgrade. I also review that the check of the credential type was created on 5.3.3, so makes sense why it wasn't happening this to me before. Thanks in advance