Make all features in Altair thread-safe

[binste]

What is your suggestion?

Right now (version 5.4.0), Altair is mostly safe to use in a threading context. However, we could go through the whole library to make sure everything is covered and then advertise this. Features which might require adjustments/locks:

• Global theme settings, e.g. if threads are switched within a with alt.themes.enable statement. Related: Make it easier for downstream libraries to *safely* contribute themes #3586
• Global renderer settings, same as for themes above
• We have a global counter we use for parameter names. Maybe this is fine. We would need to think it through, but there are also other reasons for getting rid of this, see Use object specific counter instead of a global counter for unnamed parameter & view names #3416.

Have you considered any alternative solutions?

This is not urgent and we can leave it as-is. Users and developers of other libraries can work around these limitations with locks. But it would be easier if we do it in Altair itself.

[dangotbanned]

If the stdlib considers this a thread safe counter, maybe that is an easy solution?

https://github.com/python/cpython/blob/6e06e01881dcffbeef5baac0c112ffb14cfa0b27/Lib/asyncio/tasks.py#L31-L34

# Helper to generate new task names
# This uses itertools.count() instead of a "+= 1" operation because the latter
# is not thread safe. See bpo-11866 for a longer explanation.
_task_name_counter = itertools.count(1).__next__

---

An example where I adapted this:

Code block

import itertools
from collections.abc import Callable
from typing import TYPE_CHECKING, Any, ClassVar, Protocol, runtime_checkable

_counter_glob = itertools.count(1).__next__


@runtime_checkable
class _AsyncNamed(Protocol):
    # Mixin providing uniquely named instances.
    _counter_cls: ClassVar[Callable[[], int]]
    _name: str

    def __init_subclass__(cls, *args: Any, **kwargs: Any) -> None:
        super().__init_subclass__(*args, **kwargs)
        cls._counter_cls = itertools.count(1).__next__

    def __init__(self) -> None:
        cls = type(self)
        self._name = f"{cls.__name__}-{cls._counter_cls()}-{_counter_glob()}"

    def _name_sub(self, obj: Any, /) -> str:
        other = obj.name if isinstance(obj, _AsyncNamed) else type(obj).__name__
        return f"{self.name} > {other}"

    @property
    def name(self) -> str:
        return self._name