BUG: missed dependency on tornado

[e-kwsm]

This is a pyzmq bug

• This is a pyzmq-specific bug, not an issue of zmq socket behavior. Don't worry if you're not sure! We'll figure it out together.

What pyzmq version?

26.4.0

What libzmq version?

4.3.5

Python version (and how it was installed)

python 3.13.3 as provided by pacman

OS

EndeavourOS

What happened?

Install pyzmq:

python3 -m venv $(mktemp -d)
. $_/bin/activate
pip install pyzmq

Then

>>> import zmq.eventloop
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    import zmq.eventloop
  File "/tmp/tmp.1FTlDHpLOz/lib/python3.13/site-packages/zmq/eventloop/__init__.py", line 3, in <module>
    from tornado.ioloop import IOLoop
ModuleNotFoundError: No module named 'tornado'

Code to reproduce bug

import zmq.eventloop

Traceback, if applicable

More info

No response

[minrk]

Tornado is not a hard dependency of pyzmq, it is an optional dependency of pyzmq when used with tornado. So whatever code is making this import presumably also has a direct dependency on tornado itself.

[e-kwsm]

At the moment tornado is missing in pyproject; optional dependency can be specified by e.g.

[project.optional-dependencies]
all = ["tornado"]

[minrk]

I don't feel the need to express a dependency on a project for the compatibility with that project. Pyzmq cannot be used with tornado without the application using pyzmq also having a direct dependency on tornado, so that's where the dependency belongs, imo.

[e-kwsm]

The dependency is not always explicit—a script may just import zmq.eventloop.

Also, at the moment pyzmq does not specifies tornado version requirement, and tests with only the latest version?

https://github.com/zeromq/pyzmq/blob/a37d49db40f1836e3f72297b136b87f0b05ab2e2/docs/source/changelog.md#2500

Remove support for tornado 4

Does this mean pyzmq requires tornado>=5.0?

At least test assumes tornado>=4.2:

pyzmq/tests/test_future.py

Line 24 in a37d49d

self.loop = IOLoop(make_current=False)

[minrk]

The dependency is not always explicit—a script may just import zmq.eventloop.

Do you have an example? zmq.eventloop is explicitly for compatibility with tornado applications, so it's a bit odd to have zmq.eventloop without any tornado import. But I guess that's more possible with tornado 6 now that you can use asyncio more directly?

Does this mean pyzmq requires tornado>=5.0?

Yes, pyzmq aims to support all versions of tornado that might reasonably run on supported versions of Python. tornado 4.x has been outdated for over 7 years, and I don't think it works with current Python (it may work, but I do know it's not supported).

I don't really understand the need to be more explicit than to use pyzmq with tornado, you need to install pyzmq and tornado. It is the difference between pip install pyzmq[tornado] and pip install pyzmq tornado. The latter is shorter (all wouldn't be right, since tornado is not the only optional feature, and nobody should ever install [all] in practice, except to run pyzmq's own tests).

I take your point that applying a version constraint is a good reason to express this dependency. But the supported range is and will continue to be "effectively any usable version of tornado itself", so there is no meaningful impact of a version constraint.

[SquidDev]

Do you have an example? [...] But I guess that's more possible with tornado 6 now that you can use asyncio more directly?

I also hit this issue today, while using pyzmq with asyncio. I think this is easier to trigger than the zmq.eventloop case, as tornado is not a requirement on all operating system — I do most of my dev on Linux, so things ran fine, but crashed when run on Windows!

I don't really have any good suggestions here — you could have an asyncio extra, but that's probably easy to miss, so not actually very helpful. It might be sufficient just to call this out in the zmq.asyncio module docs?