[Nullabillity]

In CPython 2.7, yes. In CPython in general, I believe it's currently still the case, but I don't think it's guaranteed for future versions.

For Python in general, no. For example, as far as I know Jython reuses the JVM's GC (and its unreliable finalizers with it).

It's also easy to introduce accidental cycles. For one, a traceback includes a reference to every frame on the call stack, so storing that somewhere on the stack would create an unintentional cycle!

[eternityforest]

The tracebacks were a lot of what made me cut back on using weakrefs and trying to make things manage their resources automatically.

Now I use close() methods for anything that needs to be closed. If I mess up and there's some obscure bug, hopefully GC will fix it, but it seems too brittle and easy to make mistakes with to rely on.