[ruby-core:109803] [Ruby master Feature#18951] Object#with to set and restore attributes around a block

Issue #18951 has been updated by austin (Austin Ziegler).


Dan0042 (Daniel DeLorme) wrote in #note-10:
> Also really like the idea, also ambivalent about such a short and generic name.
> 
> I think it would be good if the name emphasized a bit more that the original values will be restored after the block. Something in the vein of `temporarily_with`

- `#override_with`
- `#mut`
- `#mutate`
- `#borrow_with`
- `#with_restore`
- `#restore_do`
- `#tap_restore`
- `#tap_reset`
- `#override_reset`
- `#overlay_with`
- `#overlay`
- `#overlay_reset`

This could do something with `Object#extend` or maybe a temporary refinement (I have yet to *use* refinements, so I’m going to avoid trying to make an example that way).

```ruby
module RestorableOverlay
  def overlay(values)
   (@__restoreable_overlay__ ||= []) << {}
    values.each_pair { |k, v|
      @__restorable_overlay__[-1][k] = send(:"#{k}")
      send(:"#{k}=", v)
    } 
  end

  def overlay_commit
    @__restorable_overlay__.pop
  end

  def overlay_rollback
    overlay_commit.each_pair { |k, v| send(:"#{k}=", v) }
    # magic function that doesn’t exist
    if @__restorable_overlay__.empty?
      unextend RestorableOverlay
      instance_variable_del(:@__restorable_overlay__)
    end
  end
end

class Object
  def overlay_with(values)
    extend(RestorableOverlay) unless extended_with?(RestorableOverlay)
    if block_given?
      begin
        overlay(values)
        yield
      ensure
        overlay_rollback
      end
    else
     overlay(values)
    end
  end
end
```

I had done something *similar* back in 2004 while working with PDF::Writer (Ruby 1.8 mostly), originally released as https://github.com/halostatue/transaction-simple. An absolute memory hog, broke parent / child ownership cycles, and slow as can be, but it *worked* for the most part.

Maybe worth revisiting?

----------------------------------------
Feature #18951: Object#with to set and restore attributes around a block
https://bugs.ruby-lang.org/issues/18951#change-99044

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
### Use case

A very common pattern in Ruby, especially in testing is to save the value of an attribute, set a new value, and then restore the old value in an `ensure` clause.

e.g. in unit tests

```ruby
def test_something_when_enabled
  enabled_was, SomeLibrary.enabled = SomeLibrary.enabled, true
  # test things
ensure
  SomeLibrary.enabled = enabled_was
end
```

Or sometime in actual APIs:

```ruby
def with_something_enabled
  enabled_was = @enabled
  @enabled = true
  yield
ensure
  @enabled = enabled_was
end
```

There is no inherent problem with this pattern, but it can be easy to make a mistake, for instance the unit test example:

```ruby
def test_something_when_enabled
  some_call_that_may_raise
  enabled_was, SomeLibrary.enabled = SomeLibrary.enabled, true
  # test things
ensure
  SomeLibrary.enabled = enabled_was
end
```

In the above if `some_call_that_may_raise` actually raises, `SomeLibrary.enabled` is set back to `nil` rather than its original value. I've seen this mistake quite frequently.

### Proposal

I think it would be very useful to have a method on Object to implement this pattern in a correct and easy to use way. The naive Ruby implementation would be:

```ruby
class Object
  def with(**attributes)
    old_values = {}
    attributes.each_key do |key|
      old_values[key] = public_send(key)
    end
    begin
      attributes.each do |key, value|
        public_send("#{key}=", value)
      end
      yield
    ensure
      old_values.each do |key, old_value|
        public_send("#{key}=", old_value)
      end
    end
  end
end
```

NB: `public_send` is used because I don't think such method should be usable if the accessors are private.

With usage:

```ruby
def test_something_when_enabled
  SomeLibrary.with(enabled: true) do
    # test things
  end
end
```

```ruby
GC.with(measure_total_time: true, auto_compact: false) do
  # do something
end
```

### Alternate names and signatures

If `#with` isn't good, I can also think of:

  - `Object#set`
  - `Object#apply`

But the `with_` prefix is by far the most used one when implementing methods that follow this pattern.

Also if accepting a Hash is dimmed too much, alternative signatures could be:

  - `Object#set(attr_name, value)`
  - `Object#set(attr1, value1, [attr2, value2], ...)`





-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>