Issue #18951 has been updated by byroot (Jean Boussier). Thank you Matz. If it's not desired in ruby-core, I can add it to Active Support, that's no problem. ---------------------------------------- Feature #18951: Object#with to set and restore attributes around a block https://bugs.ruby-lang.org/issues/18951#change-100384 * 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], ...)` # Some real world code example that could be simplified with method - `redis-client` `with_timeout` https://github.com/redis-rb/redis-client/blob/23a5c1e2ff688518904f206df8d4a8... - Lots of tests in Rails's codebase: - Changing `Thread.report_on_exception`: https://github.com/rails/rails/blob/2d2fdc941e7497ca77f99ce5ad404b6e58f043ef... - Changing a class attribute: https://github.com/rails/rails/blob/2d2fdc941e7497ca77f99ce5ad404b6e58f043ef... -- https://bugs.ruby-lang.org/