Still confused about reference counting

Question

Out of curiosity, I am trying to understand how reference counting works in Python. These two entries:

• Why are Python Ref Counts to small integers surprisingly high?
• Is there a way to get the current ref count of an object in Python?

were helpful, but still raised questions.

1. Using sys.getrefcount() returns a different value than len(gc.get_referrers()). For example:

   >>> a = 3
   >>> print sys.getrefcount(a)
   38
   >>> print len(gc.get_referrers(a))
   23
   

   Why the difference?

2. As I understand it, the reference count on a is so high because there is already an object holding an integer value of 3 at the time I bound the name a to it. How does Python keep track of which object is holding 3 so that it binds the name a to it and increments its reference count accordingly?

Answer 1

1. gc.get_referrers only returns objects the cycle-detecting GC knows about. Objects that couldn't possibly be involved in a reference cycle don't need to be tracked by the cycle detector, so they might not show up in the get_referrers list.

2. With this array here:

   static PyIntObject *small_ints[NSMALLNEGINTS + NSMALLPOSINTS];