How do I correctly clean up a Python object

Question

class Package      def   init   self           self files                      def   del   self           for file in self files              os unlink file      del   self  above fails with an AttributeError exception   I understand Python doesn t guarantee the existence of  global variables   member data in this context   when   del     is invoked   If that is the case and this is the reason for the exception  how do I make sure the object destructs properly

User · Answer

Here is a minimal working skeleton   class SkeletonFixture       def   init   self           pass      def   enter   self           return self      def   exit   self  exc type  exc value  traceback           pass      def method self           pass   with SkeletonFixture   as fixture      fixture method     Important  return self    If you re like me  and overlook the return self part  of Clint Miller s correct answer   you will be staring at this nonsense   Traceback  most recent call last     File  tests simplestpossible py   line 17  in  lt module gt                                                                                                                                                                fixture method                                                                                                                                                                                                 AttributeError   NoneType  object has no attribute  method    Hope it helps the next person

User · Answer

atexit register is the standard way as has already been mentioned in ostrakach s answer  However  it must be noted that the order in which objects might get deleted cannot be relied upon as shown in example below  import atexit  class A object        def   init   self  val           self val   val         atexit register self hello       def hello self           print self val    def hello2        a   A 10   hello2       a   A 20   Here  order seems legitimate in terms of reverse of the order in which objects were created as program gives output as   20 10  However when  in a larger program  python s garbage collection kicks in object which is out of it s lifetime would get destructed first

User · Answer

I don t think that it s possible for instance members to be removed before   del   is called  My guess would be that the reason for your particular AttributeError is somewhere else  maybe you mistakenly remove self file elsewhere    However  as the others pointed out  you should avoid using   del    The main reason for this is that instances with   del   will not be garbage collected  they will only be freed when their refcount reaches 0   Therefore  if your instances are involved in circular references  they will live in memory for as long as the application run   I may be mistaken about all this though  I d have to read the gc docs again  but I m rather sure it works like this

User · Answer

The standard way is to use atexit register     package py import atexit import os  class Package      def   init   self           self files              atexit register self cleanup       def cleanup self           print  Running cleanup              for file in self files              print  Unlinking file      format file                 os unlink file    But you should keep in mind that this will persist all created instances of Package until Python is terminated   Demo using the code above saved as package py     python  gt  gt  gt  from package import    gt  gt  gt  p   Package    gt  gt  gt  q   Package    gt  gt  gt  q files     a    b    c    gt  gt  gt  quit   Running cleanup    Unlinking file  a Unlinking file  b Unlinking file  c Running cleanup

User · Answer

Just wrap your destructor with a try except statement and it will not throw an exception if your globals are already disposed of   Edit  Try this   from weakref import proxy  class MyList list   pass  class Package      def   init   self           self   del   im func files   MyList  1 2 3 4           self files   proxy self   del   im func files       def   del   self           print self   del   im func files   It will stuff the file list in the del function that is guaranteed to exist at the time of call  The weakref proxy is to prevent Python  or yourself from deleting the self files variable somehow  if it is deleted  then it will not affect the original file list   If it is not the case that this is being deleted even though there are more references to the variable  then you can remove the proxy encapsulation

User · Answer

I think the problem could be in   init   if there is more code than shown     del   will be called even when   init   has not been executed properly or threw an exception   Source

User · Answer

As an appendix to Clint s answer  you can simplify PackageResource using contextlib contextmanager    contextlib contextmanager def packageResource        class Package                  package   Package       yield package     package cleanup     Alternatively  though probably not as Pythonic  you can override Package   new     class Package object       def   new   cls   args    kwargs            contextlib contextmanager         def packageResource                  adapt arguments if superclass takes some              package   super Package  cls    new   cls              package   init    args    kwargs              yield package             package cleanup        def   init   self   args    kwargs                 and simply use with Package      as package   To get things shorter  name your cleanup function close and use contextlib closing  in which case you can either use the unmodified Package class via with contextlib closing Package       or override its   new   to the simpler  class Package object       def   new   cls   args    kwargs           package   super Package  cls    new   cls          package   init    args    kwargs          return contextlib closing package    And this constructor is inherited  so you can simply inherit  e g   class SubPackage Package       def close self           pass

User · Answer

A better alternative is to use weakref finalize  See the examples at Finalizer Objects and Comparing finalizers with   del     methods

User · Answer

I d recommend using Python s with statement for managing resources that need to be cleaned up   The problem with using an explicit close   statement is that you have to worry about people forgetting to call it at all or forgetting to place it in a finally block to prevent a resource leak when an exception occurs   To use the with statement  create a class with the following methods     def   enter   self    def   exit   self  exc type  exc value  traceback    In your example above  you d use   class Package      def   init   self           self files           def   enter   self           return self                 def   exit   self  exc type  exc value  traceback           for file in self files              os unlink file    Then  when someone wanted to use your class  they d do the following   with Package   as package obj        use package obj   The variable package obj will be an instance of type Package  it s the value returned by the   enter   method    Its   exit   method will automatically be called  regardless of whether or not an exception occurs   You could even take this approach a step further   In the example above  someone could still instantiate Package using its constructor without using the with clause   You don t want that to happen   You can fix this by creating a PackageResource class that defines the   enter   and   exit   methods   Then  the Package class would be defined strictly inside the   enter   method and returned   That way  the caller never could instantiate the Package class without using a with statement   class PackageResource      def   enter   self           class Package                          self package obj   Package           return self package obj      def   exit   self  exc type  exc value  traceback           self package obj cleanup     You d use this as follows   with PackageResource   as package obj        use package obj

User · Answer

It seems that the idiomatic way to do this is to provide a close   method  or similar   and call it explicitely

[python] How do I correctly clean up a Python object?

Examples related to python

Examples related to destructor