[Python-Dev] Garbage collecting closures

Phillip J. Eby pje@telecommunity.com
Mon, 14 Apr 2003 11:52:16 -0400

 >Then, that gets clarified to
 >"unless they have reference cycles, in which case they may get destroyed
 >arbitrarily later" and now "or they are used in a function containing
 >another function, which will cause a circular reference involving all
 >local variables."

Actually, the issue is that *recursive* nested functions create a circular 
reference.  Note that the body of function 'foo' contains a reference to 
'foo'.  *That* is the circular reference.

If I understand correctly, it should also be breakable by deleting 'foo' 
from the outer function when you're done with it.  E.g.:

def bar(a):
      def foo():
          return None
          x = a

      del foo   # clears the cell and breaks the cycle

Strangely, I could have sworn that there was documentation that came out 
when nested scopes were introduced that discussed this issue.  But I just 
looked at PEP 227 and the related "What's New" document, and neither 
explicitly mentions that defining recursive nested functions creates a 
circular reference.  I think I just "knew" that it would do so, from what 
*is* said in those documents and what little I knew about how the cells 
mechanism was supposed to work.

Since both PEP 227 and the What's New document mention recursive nested 
functions as a motivating example for nested scopes, perhaps they should 
mention the circular reference consequence of doing so.