How to force deletion of a python object

Question

I am curious about the details of   del   in python  when and why it should be used and what it shouldn t be used for   I ve learned the hard way that it is not really like what one would naively expected from a destructor  in that it is not the opposite of   new       init       class Foo object        def   init   self           self bar   None      def open self           if self bar     open               print  opening the bar              self bar    open       def close self           if self bar     closed               print  closing the bar              self bar    close       def   del   self           self close    if   name         main         foo   Foo       foo open       del foo     import gc     gc collect     I saw in the documentation that it is not guaranteed   del     methods are called for objects that still exist when the interpreter exits      how can it be guaranteed that for any Foo instances existing when the interpreter exits  the bar is closed  in the code snippet above does the bar get closed on del foo or on gc collect      or neither   if you want finer control of those details  e g  the bar should be closed when the object is unreferenced  what is the usual way to implement that  when   del   is called is it guaranteed that   init   has already been called   what about if the   init   raised

User · Answer

Perhaps you are looking for a context manager    gt  gt  gt  class Foo object         def   init   self           self bar   None       def   enter   self           if self bar     open             print  opening the bar            self bar    open        def   exit   self  type   value  traceback           if self bar     closed             print  closing the bar   type   value  traceback           self bar    close        gt  gt  gt    gt  gt  gt  with Foo   as f            oh no something crashes the program         sys exit 0       opening the bar closing the bar  lt type  exceptions SystemExit  gt  0  lt traceback object at 0xb7720cfc gt

User · Answer

The way to close resources are context managers  aka the with statement   class Foo object      def   init   self       self bar   None    def   enter   self       if self bar     open         print  opening the bar        self bar    open      return self   this is bound to the  as  part    def close self       if self bar     closed         print  closing the bar        self bar    close     def   exit   self   err       self close    if   name         main       with Foo   as foo      print foo  foo bar   output   opening the bar  lt   main   Foo object at 0x17079d0 gt  open closing the bar     2  Python s objects get deleted when their reference count is 0  In your example the del foo removes the last reference so   del   is called instantly  The GC has no part in this   class Foo object        def   del   self           print  deling   self  if   name         main         import gc     gc disable     no gc     f   Foo       print  before      del f   f gets deleted right away     print  after    output   before deling  lt   main   Foo object at 0xc49690 gt  after   The gc has nothing to do with deleting your and most other objects  It s there to clean up when simple reference counting does not work  because of self-references or circular references   class Foo object       def   init   self  other None             make a circular reference         self link   other         if other is not None              other link   self      def   del   self           print  deling   self  if   name         main         import gc     gc disable          f   Foo Foo        print  before      del f   nothing gets deleted here     print  after      gc collect       print gc garbage   The GC knows the two Foos are garbage  but won t delete                        them because they have a   del   method     print  after gc        break up the cycle and delete the reference from gc garbage     del gc garbage 0  link  gc garbage        print  done    output   before after   lt   main   Foo object at 0x22ed8d0 gt    lt   main   Foo object at 0x22ed950 gt   after gc deling  lt   main   Foo object at 0x22ed950 gt  deling  lt   main   Foo object at 0x22ed8d0 gt  done     3  Lets see   class Foo object       def   init   self            raise Exception      def   del   self           print  deling   self  if   name         main         f   Foo     gives   Traceback  most recent call last     File  asd py   line 10  in  lt module gt      f   Foo     File  asd py   line 4  in   init       raise Exception Exception deling  lt   main   Foo object at 0xa3a910 gt    Objects are created with   new   then passed to   init   as self  After a exception in   init    the object will typically not have a name  ie the f   part isn t run  so their ref count is 0  This means that the object is deleted normally and   del   is called

User · Answer

In general  to make sure something happens no matter what  you use  from exceptions import NameError  try      f   open x  except ErrorType as e      pass   handle the error finally      try          f close       except NameError  pass   finally blocks will be run whether or not there is an error in the try block  and whether or not there is an error in any error handling that takes place in except blocks  If you don t handle an exception that is raised  it will still be raised after the finally block is excecuted   The general way to make sure a file is closed is to use a  context manager    http   docs python org reference datamodel html context-managers  with open x  as f        do stuff   This will automatically close f   For your question  2  bar gets closed on immediately when it s reference count reaches zero  so on del foo if there are no other references   Objects are NOT created by   init    they re created by   new     http   docs python org reference datamodel html object new  When you do foo   Foo   two things are actually happening  first a new object is being created    new    then it is being initialized    init    So there is no way you could possibly call del foo before both those steps have taken place  However  if there is an error in   init      del   will still be called because the object was actually already created in   new     Edit  Corrected when deletion happens if a reference count decreases to zero

User · Answer

Add an exit handler that closes all the bars    del     gets called when the number of references to an object hits 0 while the VM is still running  This may be caused by the GC  If   init     raises an exception then the object is assumed to be incomplete and   del     won t be invoked

[python] How to force deletion of a python object?

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