Question

When are objects garbage collected in python? When is the memory released and does the collection impact performance? Can one opt out or tune the gc algorithm and if so how?

Answer 1

Here is an excerpt from the language reference

Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether — it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.

CPython implementation detail: CPython currently uses a reference-counting scheme with (optional) delayed detection of cyclically linked garbage, which collects most objects as soon as they become unreachable, but is not guaranteed to collect garbage containing circular references. See the documentation of the gc module for information on controlling the collection of cyclic garbage. Other implementations act differently and CPython may change. Do not depend on immediate finalization of objects when they become unreachable (ex: always close files).

EDIT: About postponing garbage collection …. the gc module allows you to interact with the garbage collector, and disable it if you want to and change collection frequency etc. But I have not used it myself. Also, cycles that contain any objects with __del__ methods are not collected.