bpo-32377: improve __del__ docs and fix mention about resurrection (GH-4927) (#4929)

miss-islington · pitrou · commit dc5770b161a5 · 2017-12-19T20:00:13.000+01:00
* Fix GH-32377: improve __del__ docs and fix mention about resurrection * Mention that CPython only calls __del__ once. (cherry picked from commit 4b96593)
diff --git a/Doc/glossary.rst b/Doc/glossary.rst
@@ -391,7 +391,8 @@ Glossary
    garbage collection
       The process of freeing memory when it is not used anymore.  Python
       performs garbage collection via reference counting and a cyclic garbage
-      collector that is able to detect and break reference cycles.
+      collector that is able to detect and break reference cycles.  The
+      garbage collector can be controlled using the :mod:`gc` module.
 
       .. index:: single: generator
 
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
@@ -1157,61 +1157,69 @@ Basic customization
 
    .. index::
       single: destructor
+      single: finalizer
       statement: del
 
    Called when the instance is about to be destroyed.  This is also called a
-   destructor.  If a base class has a :meth:`__del__` method, the derived class's
-   :meth:`__del__` method, if any, must explicitly call it to ensure proper
-   deletion of the base class part of the instance.  Note that it is possible
-   (though not recommended!) for the :meth:`__del__` method to postpone destruction
-   of the instance by creating a new reference to it.  It may then be called at a
-   later time when this new reference is deleted.  It is not guaranteed that
-   :meth:`__del__` methods are called for objects that still exist when the
-   interpreter exits.
+   finalizer or (improperly) a destructor.  If a base class has a
+   :meth:`__del__` method, the derived class's :meth:`__del__` method,
+   if any, must explicitly call it to ensure proper deletion of the base
+   class part of the instance.
+
+   It is possible (though not recommended!) for the :meth:`__del__` method
+   to postpone destruction of the instance by creating a new reference to
+   it.  This is called object *resurrection*.  It is implementation-dependent
+   whether :meth:`__del__` is called a second time when a resurrected object
+   is about to be destroyed; the current :term:`CPython` implementation
+   only calls it once.
+
+   It is not guaranteed that :meth:`__del__` methods are called for objects
+   that still exist when the interpreter exits.
 
    .. note::
 
       ``del x`` doesn't directly call ``x.__del__()`` --- the former decrements
       the reference count for ``x`` by one, and the latter is only called when
-      ``x``'s reference count reaches zero.  Some common situations that may
-      prevent the reference count of an object from going to zero include:
-      circular references between objects (e.g., a doubly-linked list or a tree
-      data structure with parent and child pointers); a reference to the object
-      on the stack frame of a function that caught an exception (the traceback
-      stored in ``sys.exc_info()[2]`` keeps the stack frame alive); or a
-      reference to the object on the stack frame that raised an unhandled
-      exception in interactive mode (the traceback stored in
-      ``sys.last_traceback`` keeps the stack frame alive).  The first situation
-      can only be remedied by explicitly breaking the cycles; the second can be
-      resolved by freeing the reference to the traceback object when it is no
-      longer useful, and the third can be resolved by storing ``None`` in
-      ``sys.last_traceback``.
-      Circular references which are garbage are detected and cleaned up when
-      the cyclic garbage collector is enabled (it's on by default). Refer to the
-      documentation for the :mod:`gc` module for more information about this
-      topic.
+      ``x``'s reference count reaches zero.
+
+   .. impl-detail::
+      It is possible for a reference cycle to prevent the reference count
+      of an object from going to zero.  In this case, the cycle will be
+      later detected and deleted by the :term:`cyclic garbage collector
+      <garbage collection>`.  A common cause of reference cycles is when
+      an exception has been caught in a local variable.  The frame's
+      locals then reference the exception, which references its own
+      traceback, which references the locals of all frames caught in the
+      traceback.
+
+      .. seealso::
+         Documentation for the :mod:`gc` module.
 
    .. warning::
 
       Due to the precarious circumstances under which :meth:`__del__` methods are
       invoked, exceptions that occur during their execution are ignored, and a warning
-      is printed to ``sys.stderr`` instead.  Also, when :meth:`__del__` is invoked in
-      response to a module being deleted (e.g., when execution of the program is
-      done), other globals referenced by the :meth:`__del__` method may already have
-      been deleted or in the process of being torn down (e.g. the import
-      machinery shutting down).  For this reason, :meth:`__del__` methods
-      should do the absolute
-      minimum needed to maintain external invariants.  Starting with version 1.5,
-      Python guarantees that globals whose name begins with a single underscore are
-      deleted from their module before other globals are deleted; if no other
-      references to such globals exist, this may help in assuring that imported
-      modules are still available at the time when the :meth:`__del__` method is
-      called.
+      is printed to ``sys.stderr`` instead.  In particular:
 
-      .. index::
-         single: repr() (built-in function); __repr__() (object method)
+      * :meth:`__del__` can be invoked when arbitrary code is being executed,
+        including from any arbitrary thread.  If :meth:`__del__` needs to take
+        a lock or invoke any other blocking resource, it may deadlock as
+        the resource may already be taken by the code that gets interrupted
+        to execute :meth:`__del__`.
+
+      * :meth:`__del__` can be executed during interpreter shutdown.  As a
+        consequence, the global variables it needs to access (including other
+        modules) may already have been deleted or set to ``None``. Python
+        guarantees that globals whose name begins with a single underscore
+        are deleted from their module before other globals are deleted; if
+        no other references to such globals exist, this may help in assuring
+        that imported modules are still available at the time when the
+        :meth:`__del__` method is called.
 
 
+   .. index::
+      single: repr() (built-in function); __repr__() (object method)
+
 .. method:: object.__repr__(self)
 
    Called by the :func:`repr` built-in function to compute the "official" string
