> So please explain me :-).

it sounded like you did not really want any explanation 

>  os.urandom() can only be called by one thread/greenlet at a time.

do you mean that we cannot have two parallel calls of that function ?
e.g. two opened FD at the same time ?

> So I would assume that you have asymptotically at least as many open sockets than FDs open to os.urandom.

a web socket application that spawns one socket per connection, then uses a lib that calls many times os.urandom(), will generate most of its FDs on os urandom

but since you said that os.urandom() should not be used in the first place - that's what I will keep in mind