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())