When should I use uuid.uuid1() vs. uuid.uuid4() in python?
uuid1() is guaranteed to not produce any collisions (under the assumption you do not create too many of them at the same time). I wouldn't use it if it's important that there's no connection between the uuid and the computer, as the mac address gets used to make it unique across computers.
You can create duplicates by creating more than 214 uuid1 in less than 100ns, but this is not a problem for most use cases.
uuid4() generates, as you said, a random UUID. The chance of a collision is really, really, really small. Small enough, that you shouldn't worry about it. The problem is, that a bad random-number generator makes it more likely to have collisions.
This excellent answer by Bob Aman sums it up nicely. (I recommend reading the whole answer.)
Frankly, in a single application space without malicious actors, the extinction of all life on earth will occur long before you have a collision, even on a version 4 UUID, even if you're generating quite a few UUIDs per second.
One instance when you may consider uuid1() rather than uuid4() is when UUIDs are produced on separate machines, for example when multiple online transactions are process on several machines for scaling purposes.
In such a situation, the risks of having collisions due to poor choices in the way the pseudo-random number generators are initialized, for example, and also the potentially higher numbers of UUIDs produced render more likely the possibility of creating duplicate IDs.
Another interest of uuid1(), in that case is that the machine where each GUID was initially produced is implicitly recorded (in the "node" part of UUID). This and the time info, may help if only with debugging.
My team just ran into trouble using UUID1 for a database upgrade script where we generated ~120k UUIDs within a couple of minutes. The UUID collision led to violation of a primary key constraint.
We've upgraded 100s of servers but on our Amazon EC2 instances we ran into this issue a few times. I suspect poor clock resolution and switching to UUID4 solved it for us.