Issue #13893 has been updated by Eregon (Benoit Daloze).


One consideration: `Fiber.current` is not available until `require "fiber"`.
So an API based on `Fiber.current` doesn't seem nice while `require "fiber"` is needed (will be NoMethodError + confusion if not required).

I think it would be too incompatible to change Thread.current#[]/[]= to anything else.
It would need first a deprecation (but so many usages it's going to be very annoying), then removal for some years, then adding back as thread-locals.
As such Fiber#[]/[]= don't seem nice, because they would just be redundant with Thread.current#[]/[]= or make it even more confusing.

The `.local` API seems nice and avoid those issues.

`Fiber.local` wouldn't need any check, but `Fiber.current.local` would need checks.
That's suboptimal as `Fiber.current.local` will need to lookup the current Fiber/Thread twice instead of once.
On Rubies without a GIL or with Ractors, `Thread.current` and `Fiber.current` will be on its their some kind of (native) thread-local lookup.
Also `Fiber.current.local` needs Fiber.current has the `require "fiber"` issue mentioned above.

----------------------------------------
Feature #13893: Add Fiber#[] and Fiber#[]= and restore Thread#[] and Thread#[]= to their original behavior
https://bugs.ruby-lang.org/issues/13893#change-86997

* Author: cremes (Chuck Remes)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby 3 API cleanup suggestion.

The Thread and Fiber classes have a very odd API for setting/getting thread local and fiber local variables. With Ruby 3 coming soon, this is a perfect opportunity to make this API more coherent and return to the Principal of Least Surprise. The concept of Fibers and Threads should be completely separated and we should no longer assume that a Fiber is attached to any particular Thread.

I suggest this:

```
class Fiber
  # Gets a fiber-local variable.
  def [](index)
    ...
  end

  # Sets a fiber-local variable.
  def []=(index, value)
    ...
  end

  # Returns true if the given +key+ exists as a fiber-local variable.
  def key?(key)
    ...
  end

  # Returns an array of fiber-local variable names as symbols.
  def keys
    ...
  end
end

class Thread
  # Gets a thread-local variable.
  def [](index)
    ...
  end

  # Sets a thread-local variable.
  def []=(index, value)
    ...
  end

  # Returns true if the given +key+ exists as a thread-local variable.
  def key?(key)
    ...
  end

  # Returns an array of thread-local variable names as symbols.
  def keys
    ...
  end
end
```

Also, remove ```Thread#thread_variable?```, `Thread#thread_variable_get`, `Thread#variable_set`, and `Thread#thread_variables` since that behavior is already covered by `Thread#key?`, `Thread#keys`, `Thread#[]`, and `Thread#[]=`. The APIs for both Thread and Fiber are more coherent and less surprising with these changes.



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