Call in prefect._internal.concurrency.calls raises AttributeError on equality

[willhcr]

Bug summary

Call objects stored in contextvars raise AttributeError when compared using != after internal attributes (args, kwargs) have been deleted. This code del self.args, self.kwargs is run in a few places in Call. The error occurs because the class uses an auto-generated __eq__ from @dataclass, which accesses these now-missing fields during comparison.

This led to an error when using async code with prefect and Django in asgiref. Fixing this would allow Prefect tasks and Django async ORM methods to interoperate.

@task
async def get_account(account_uuid: UUID):
    return await Account.objects.select_related('domain').aget(uuid=account_uuid)

Exception from asgiref:

def _restore_context(context: contextvars.Context) -> None:
    # Check for changes in contextvars, and set them to the current
    # context for downstream consumers
    for cvar in context:
        cvalue = context.get(cvar)
        try:
            if cvar.get() != cvalue:
                cvar.set(cvalue)
        except LookupError:
            cvar.set(cvalue)

  if cvar.get() != cvalue:
AttributeError: 'Call' object has no attribute 'args'

Version info

Version:             3.3.7
API version:         0.8.4
Python version:      3.12.10
Git commit:          8f86aaee
Built:               Mon, Apr 28, 2025 03:04 PM
OS/Arch:             linux/x86_64
Profile:             ephemeral
Server type:         ephemeral
Pydantic version:    2.10.6
Server:
  Database:          sqlite
  SQLite version:    3.47.1
Integrations:
  prefect-azure:     0.4.4
  prefect-docker:    0.6.4

Additional context

Implementing object equality resolved this issue. But is attribute level comparison used elsewhere with Call?

class Call(Generic[T]):
...
    def __eq__(self, other):
        return self is other

[willhcr]

Or perhaps one of these approaches would be right:

def __eq__(self, other):
    if not isinstance(other, Call):
        return NotImplemented

    try:
        self_args = self.args
        self_kwargs = self.kwargs
        other_args = other.args
        other_kwargs = other.kwargs
    except AttributeError:
        # Option 1: fallback to identity comparison
        return self is other

        # Option 2: treat dropped Calls as always unequal
        # return False

    return (
        self.future == other.future and
        self.fn == other.fn and
        self_args == other_args and
        self_kwargs == other_kwargs and
        self.context == other.context and
        self.timeout == other.timeout and
        self.runner == other.runner
    )

[GalLadislav]

Hi, this is something i would love to have resolved too. I would suggest to reasign self.args and self.kwargs to empty objects (after the del call) and then use internal flag to signal that the values were dropped. But i am not sure where in the code it relies on AttributeError being raised other than in Call.__repr__.

I can create the PR if this suggestion (or resolving the issue) is approved.

[zzstoatzz]

hi @willhcr and @GalLadislav - thanks for the detail! I've started on this here: #18110

we generally want to be careful with changes to _internal/concurrency as it underpins quite a lot, so we'll plan to test it a bit more thoroughly. any poking/tests you want to do there would be appreciated!

[willhcr]

I've tested this branch on my codebase. My test suite passes (33 tests using prefect_test_harness), including ones using DJango async ORM methods which didn't work previously. I also ran a test flow, with 14 subflows, 131 task runs, mostly sync tasks using .submit() to a TaskRunner, and all worked as expected.

[willhcr]

While testing, I've discovered the original error reliably does not occur on Python 3.13 even without this fix. In contrast, the error reliably occurs on Python 3.12.

Python 3.13 in Docker container docker.io/library/python:3.13-slim

No error

>>> sys.version
'3.13.3 (main, May  9 2025, 23:48:58) [GCC 12.2.0]'

# prefect version
Version:             3.4.2
API version:         0.8.4
Python version:      3.13.3
Git commit:          c3c1c119
Built:               Mon, May 19, 2025 04:04 PM
OS/Arch:             linux/x86_64
Profile:             ephemeral
Server type:         server
Pydantic version:    2.11.4
Integrations:
  prefect-azure:     0.4.4
  prefect-docker:    0.6.5

Python 3.12 in Docker container docker.io/library/python:3.12-slim

AttributeError: 'Call' object has no attribute 'args'

>>> sys.version
'3.12.9 (main, Feb  6 2025, 22:37:05) [GCC 12.2.0]'

# prefect version
Version:             3.4.2
API version:         0.8.4
Python version:      3.12.9
Git commit:          c3c1c119
Built:               Mon, May 19, 2025 04:04 PM
OS/Arch:             linux/x86_64
Profile:             ephemeral
Server type:         server
Pydantic version:    2.11.4
Integrations:
  prefect-azure:     0.4.4
  prefect-docker:    0.6.5

Standalone Python 3.12 from uv

AttributeError: 'Call' object has no attribute 'args'

>>> sys.version
'3.12.10 (main, Apr  9 2025, 04:03:51) [Clang 20.1.0 ]'

# prefect version
Version:             3.4.2
API version:         0.8.4
Python version:      3.12.10
Git commit:          c3c1c119
Built:               Mon, May 19, 2025 04:04 PM
OS/Arch:             linux/x86_64
Profile:             ephemeral
Server type:         ephemeral
Pydantic version:    2.11.4
Server:
  Database:          sqlite
  SQLite version:    3.47.1
Integrations:
  prefect-azure:     0.4.4
  prefect-docker:    0.6.5

[willhcr]

Python 3.13 changed the semantics of dataclass equality. This change is still needed for 3.12 and below.

[zzstoatzz]

sounds like good news! I appreciate you testing @willhcr - we'll get this cleaned up, merged and released soon