Issue #18951 has been updated by byroot (Jean Boussier).
Status changed from Open to Rejected
Marking this as rejected.
Will be in Active Support 7.1
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Feature #18951: Object#with to set and restore attributes around a block
https://bugs.ruby-lang.org/issues/18951#change-102156
* Author: byroot (Jean Boussier)
* Status: Rejected
* Priority: Normal
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### 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/23a5c1e2ff688518904f206df8d4a…
- Lots of tests in Rails's codebase:
- Changing `Thread.report_on_exception`:
https://github.com/rails/rails/blob/2d2fdc941e7497ca77f99ce5ad404b6e58f043e…
- Changing a class attribute:
https://github.com/rails/rails/blob/2d2fdc941e7497ca77f99ce5ad404b6e58f043e…
--
https://bugs.ruby-lang.org/