1  """
       2  gc.get_referrers() can be used to see objects before they are fully built.
       3  
       4  Note that this is only an example.  There are many ways to crash Python
       5  by using gc.get_referrers(), as well as many extension modules (even
       6  when they are using perfectly documented patterns to build objects).
       7  
       8  Identifying and removing all places that expose to the GC a
       9  partially-built object is a long-term project.  A patch was proposed on
      10  SF specifically for this example but I consider fixing just this single
      11  example a bit pointless (#1517042).
      12  
      13  A fix would include a whole-scale code review, possibly with an API
      14  change to decouple object creation and GC registration, and according
      15  fixes to the documentation for extension module writers.  It's unlikely
      16  to happen, though.  So this is currently classified as
      17  "gc.get_referrers() is dangerous, use only for debugging".
      18  """
      19  
      20  import gc
      21  
      22  
      23  def g():
      24      marker = object()
      25      yield marker
      26      # now the marker is in the tuple being constructed
      27      [tup] = [x for x in gc.get_referrers(marker) if type(x) is tuple]
      28      print(tup)
      29      print(tup[1])
      30  
      31  
      32  tuple(g())