threading.Condition vs threading.Event

Simply put, you use a Condition when threads are interested in waiting for something to become true, and once its true, to have exclusive access to some shared resource.

Whereas you use an Event when threads are just interested in waiting for something to become true.

In essence, Condition is an abstracted Event + Lock, but it gets more interesting when you consider that you can have several different Conditions over the same underlying lock. Thus you could have different Conditions describing the state of the underlying resource meaning you can wake workers that are only interested in particular states of the shared resource.

Another subtle difference is that Event's set() affects future calls of wait() (that is, subsequent calls of wait() will return True and won't block until clear() is called), whereas Condition's notify() (or notify_all()) doesn't (subsequent calls of wait() will block till next call of notify()).