> - you replaced the _AcquireFutures context manager on all futures with an >individual lock. In my opinion, it should be changed in a different patch. I >don't know which option is better, but if it is changed, it should be changed >in the whole file.

I can pursue the replacement of _AcquireFutures in a different patch, however I don't necessarily agree that it should be changed in the whole file. as_completed() is different than wait(), since wait() is obligated to return done and notdone Futures, which might require a different implementation. Initially, it may be better to only change as_completed(), where there is a clear concurrency win.