What is the del method How to call it

Question

I am reading a code  There is a class in which   del   method is defined  I figured out that this method is used to destroy an instance of the class  However  I cannot find a place where this method is used  The main reason for that is that I do not know how this method is used  probably not like that  obj1 del    So  my questions is how to call the   del   method

User · Answer

The   del   method  it will be called when the object is garbage collected   Note that it isn t necessarily guaranteed to be called though   The following code by itself won t necessarily do it   del obj   The reason being that del just decrements the reference count by one   If something else has a reference to the object    del   won t get called   There are a few caveats to using   del   though   Generally  they usually just aren t very useful   It sounds to me more like you want to use a close method or maybe a with statement   See the python documentation on   del   methods   One other thing to note     del   methods can inhibit garbage collection if overused   In particular  a circular reference that has more than one object with a   del   method won t get garbage collected   This is because the garbage collector doesn t know which one to call first   See the documentation on the gc module for more info

User · Answer

As mentioned earlier  the   del   functionality is somewhat unreliable  In cases where it might seem useful  consider using the   enter   and   exit   methods instead  This will give a behaviour similar to the with open   as f  pass syntax used for accessing files    enter   is automatically called when entering the scope of with  while   exit   is automatically called when exiting it  See this question for more details

User · Answer

I wrote up the answer for another question  though this is a more accurate question for it     How do constructors and destructors work   Here is a slightly opinionated answer   Don t use   del    This is not C   or a language built for destructors  The   del   method really should be gone in Python 3 x  though I m sure someone will find a use case that makes sense  If you need to use   del    be aware of the basic limitations per http   docs python org reference datamodel html      del   is called when the garbage collector happens to be collecting the objects  not when you lose the last reference to an object and not when you execute del object    del   is responsible for calling any   del   in a superclass  though it is not clear if this is in method resolution order  MRO  or just calling each superclass  Having a   del   means that the garbage collector gives up on detecting and cleaning any cyclic links  such as losing the last reference to a linked list  You can get a list of the objects ignored from gc garbage  You can sometimes use weak references to avoid the cycle altogether  This gets debated now and then  see http   mail python org pipermail python-ideas 2009-October 006194 html  The   del   function can cheat  saving a reference to an object  and stopping the garbage collection  Exceptions explicitly raised in   del   are ignored    del   complements   new   far more than   init    This gets confusing  See http   www algorithm co il blogs programming python-gotchas-1-del-is-not-the-opposite-of-init  for an explanation and gotchas    del   is not a  well-loved  child in Python  You will notice that sys exit   documentation does not specify if garbage is collected before exiting  and there are lots of odd issues  Calling the   del   on globals causes odd ordering issues  e g   http   bugs python org issue5099  Should   del   called even if the   init   fails  See http   mail python org pipermail python-dev 2000-March thread html 2423 for a long thread    But  on the other hand      del   means you do not forget to call a close statement  See http   eli thegreenplace net 2009 06 12 safely-using-destructors-in-python  for a pro   del   viewpoint  This is usually about freeing ctypes or some other special resource    And my pesonal reason for not liking the   del   function    Everytime someone brings up   del   it devolves into thirty messages of confusion  It breaks these items in the Zen of Python    Simple is better than complicated  Special cases aren t special enough to break the rules  Errors should never pass silently  In the face of ambiguity  refuse the temptation to guess  There should be one  ndash  and preferably only one  ndash  obvious way to do it  If the implementation is hard to explain  it s a bad idea     So  find a reason not to use   del

User · Answer

The   del   method  note spelling   is called when your object is finally destroyed   Technically speaking  in cPython  that is when there are no more references to your object  ie when it goes out of scope   If you want to delete your object and thus call the   del   method use  del obj1   which will delete the object  provided there weren t any other references to it    I suggest you write a small class like this  class T      def   del   self           print  deleted    And investigate in the python interpreter  eg   gt  gt  gt  a   T    gt  gt  gt  del a deleted  gt  gt  gt  a   T    gt  gt  gt  b   a  gt  gt  gt  del b  gt  gt  gt  del a deleted  gt  gt  gt  def fn            a   T           print  exiting fn       gt  gt  gt  fn   exiting fn deleted  gt  gt  gt       Note that jython and ironpython have different rules as to exactly when the object is deleted and   del   is called   It isn t considered good practice to use   del   though because of this and the fact that the object and its environment may be in an unknown state when it is called   It isn t absolutely guaranteed   del   will be called either - the interpreter can exit in various ways without deleteting all objects

User · Answer

del   is a finalizer  It is called when an object is garbage collected which happens at some point after all references to the object have been deleted    In a simple case this could be right after you say del x or  if x is a local variable  after the function ends  In particular  unless there are circular references  CPython  the standard Python implementation  will garbage collect immediately    However  this is an implementation detail of CPython  The only required property of Python garbage collection is that it happens after all references have been deleted  so this might not necessary happen right after and might not happen at all    Even more  variables can live for a long time for many reasons  e g  a propagating exception or module introspection can keep variable reference count greater than 0  Also  variable can be a part of cycle of references     CPython with garbage collection turned on breaks most  but not all  such cycles  and even then only periodically   Since you have no guarantee it s executed  one should never put the code that you need to be run into   del         instead  this code belongs to finally clause of the try block or to a context manager in a with statement  However  there are valid use cases for   del    e g  if an object X references Y and also keeps a copy of Y reference in a global cache  cache  X - gt  Y     Y  then it would be polite for X   del   to also delete the cache entry   If you know that the destructor provides  in violation of the above guideline  a required cleanup  you might want to call it directly  since there is nothing special about it as a method  x   del      Obviously  you should you do so only if you know that it doesn t mind to be called twice  Or  as a last resort  you can redefine this method using  type x    del     my safe cleanup method

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