When is del useful in Python

Question

I can t really think of any reason why Python needs the del keyword  and most languages seem to not have a similar keyword   For instance  rather than deleting a variable  one could just assign None to it  And when deleting from a dictionary  a del method could be added  Is there a reason to keep del in Python  or is it a vestige of Python s pre-garbage collection days

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To add a few points to above answers  del x  Definition of x indicates r - gt  o  a reference r pointing to an object o  but del x changes r rather than o  It is an operation on the reference  pointer  to object rather than the object associated with x  Distinguishing between r and o is key here    It removes it from locals    Removes it from globals   if x belongs there  Removes it from the stack frame  removes the reference physically from it  but the object itself resides in object pool and not in the stack frame   Removes it from the current scope  It is very useful to limit the span of definition of a local variable  which otherwise can cause problems  It is more about declaration of the name rather than definition of content  It affects where x belongs to  not where x points to  The only physical change in memory is this  For example if x is in a dictionary or list  it  as a reference  is removed from there and not necessarily from the object pool   In this example  the dictionary it belongs is the stack frame  locals     which overlaps with globals

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I think one of the reasons that del has its own syntax is that replacing it with a function might be hard in certain cases given it operates on the binding or variable and not the value it references  Thus if a function version of del were to be created a context would need to be passed in  del foo would need to become globals   remove  foo   or locals   remove  foo   which gets messy and less readable  Still I say getting rid of del would be good given its seemingly rare use  But removing language features flaws can be painful  Maybe python 4 will remove it

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del is the equivalent of  unset  in many languages and as a cross reference point moving from another language to python   people tend to look for commands that do the same thing that they used to do in their first language    also setting a var to    or none doesn t really remove the var from scope  it just empties its value the name of the var itself would still be stored in memory   why    in a memory intensive script  keeping trash behind its just a no no and anyways   every language out there has some form of an  unset delete  var function  why not python

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Force closing a file after using numpy load   A niche usage perhaps but I found it useful when using numpy load to read a file  Every once in a while I would update the file and need to copy a file with the same name to the directory   I used del to release the file and allow me to copy in the new file   Note I want to avoid the with context manager as I was playing around with plots on the command line and didn t want to be pressing tab a lot   See this question

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I would like to elaborate on the accepted answer to highlight the nuance between setting a variable to None versus removing it with del   Given the variable foo    bar   and the following function definition   def test var var       if var          print  variable tested true       else          print  variable tested false     Once initially declared  test var foo  yields variable tested true as expected   Now try   foo   None test var foo    which yields variable tested false   Contrast this behavior with   del foo test var foo    which now raises NameError  name  foo  is not defined

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I ve found del to be useful for pseudo-manual memory management when handling large data with Numpy  For example   for image name in large image set      large image   io imread image name      height  width  depth   large image shape     large mask   np all large image     lt some condition gt         Clear memory  make space     del large image  gc collect        large processed image   np zeros  height  width  depth       large processed image large mask     new value      io imsave  processed image png   large processed image         Clear memory  make space     del large mask  large processed image  gc collect     This can be the difference between bringing a script to a grinding halt as the system swaps like mad when the Python GC can t keep up  and it running perfectly smooth below a loose memory threshold that leaves plenty of headroom to use the machine to browse and code while it s working

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del is removing the variable  so that it cannot be re-initialized  Setting it to None enables you to re-initialize  a    quot python string quot          print a  del a print a  a    quot new python string quot  print a   Output  python string Traceback  most recent call last     File  quot testing py quot   line 4  in  lt module gt      print a  NameError  name  a  is not defined

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Using  del  explicitly is also better practice than assigning a variable to None  If you attempt to del a variable that doesn t exist  you ll get a runtime error but if you attempt to set a variable that doesn t exist to None  Python will silently set a new variable to None  leaving the variable you wanted deleted where it was  So del will help you catch your mistakes earlier

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As an example of what del can be used for  I find it useful i situations like this   def f a  b  c 3       return            format a  b  c   def g   kwargs       if  c  in kwargs and kwargs  c   is None          del kwargs  c        return f   kwargs     g a 1  b 2  c None       1 2 3    g a 1  b 2       1 2 3    g a 1  b 2  c 4       1 2 4    These two functions can be in different packages modules and the programmer doesn t need to know what default value argument c in f actually have  So by using kwargs in combination with del you can say  I want the default value on c  by setting it to None  or in this case also leave it    You could do the same thing with something like   def g a  b  c None       kwargs     a   a                 b   b      if c is not None          kwargs  c     c      return f   kwargs    However I find the previous example more DRY and elegant

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When is del useful in python    You can use it to remove a single element of an array instead of the slice syntax x i i 1      This may be useful if for example you are in os walk and wish to delete an element in the directory  I would not consider a keyword useful for this though  since one could just make a    remove index  method  the  remove method is actually search-and-remove-first-instance-of-value

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Every object in python has an identifier  Type  reference count associated with it  when we use del  the reference count is reduced  when the reference count becomes zero it is a potential candidate for getting garbage collected  This differentiates the del when compared to setting an identifier to None  In later case it simply means the object is just left out wild  until we are out of scope in which case the count is reduced  and simply now the identifier point to some other object memory location

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Firstly  you can del other things besides local variables  del list item 4  del dictionary  alpha     Both of which should be clearly useful  Secondly  using del on a local variable makes the intent clearer   Compare   del foo   to  foo   None   I know in the case of del foo that the intent is to remove the variable from scope  It s not clear that foo   None is doing that  If somebody just assigned foo   None I might think it was dead code  But I instantly know what somebody who codes del foo was trying to do

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One place I ve found del useful is cleaning up extraneous variables in for loops   for x in some list    do x  del x   Now you can be sure that x will be undefined if you use it outside the for loop

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Here goes my 2 cents contribution  I have a optimization problem where I use a Nlopt library for it  I initializing the class and some of its methods  I was using in several other parts of the code  I was having ramdom results even if applying the same numerical problem  I just realized that by doing it  some spurius data was contained in the object when it should have no issues at all  After using del  I guess the memory is being properly cleared and it might be an internal issue to that class where some variables might not be liking to be reused without proper constructor

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The  del  command is very useful for controlling data in an array  for example   elements     A    B    C    D     Remove first element  del elements  1  print elements    Output     B    C    D

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Yet another niche usage  In pyroot with ROOT5 or ROOT6   del  may be useful to remove a python object that referred to a no-longer existing C   object  This allows the dynamic lookup of pyroot to find an identically-named C   object and bind it to the python name  So you can have a scenario such as   import ROOT as R input file   R TFile  inputs    my file name    root   tree   input file Get  r   tree Draw  hy gt  gt hh 10 0 5    R gPad Close   R hy   shows that hy is still available  It can even be redrawn at this stage  tree Draw  hy gt  gt hh 3 0 3      overwrites the C   object in ROOT s namespace R hy   shows that R hy is None  since the C   object it pointed to is gone del R hy R hy   now finds the new C   object   Hopefully  this niche will be closed with ROOT7 s saner object management

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There s this part of what del does  from the Python Language Reference       Deletion of a name removes the binding of that name from the local or global namespace   Assigning None to a name does not remove the binding of the name from the namespace    I suppose there could be some debate about whether removing a name binding is actually useful  but that s another question

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del is often seen in   init   py files  Any global variable that is defined in an   init   py file is automatically  exported   it will be included in a from module import     One way to avoid this is to define   all    but this can get messy and not everyone uses it    For example  if you had code in   init   py like  import sys if sys version info  lt   3        print  Python 2 not supported     Then your module would export the sys name  You should instead write  import sys if sys version info  lt   3        print  Python 2 not supported    del sys

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Just another thinking   When debugging http applications in framework like Django  the call stack full of useless and messed up variables previously used  especially when it s a very long list  could be very painful for developers  so  at this point  namespace controlling could be useful

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Once I had to use   del serial serial   None   because using only   serial   None   didn t release the serial port fast enough to immediately open it again  From that lesson I learned that del really meant   GC this NOW  and wait until it s done  and that is really useful in a lot of situations  Of course  you may have a system gc del this and wait balbalbalba obj

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Deleting a variable is different than setting it to None  Deleting variable names with del is probably something used rarely  but it is something that could not trivially be achieved without a keyword  If you can create a variable name by writing a 1  it is nice that you can theoretically undo this by deleting a   It can make debugging easier in some cases as trying to access a deleted variable will raise an NameError   You can delete class instance attributes  Python lets you write something like   class A object       def set a self  a           self a a a A   a set a 3  if hasattr a   a        print  Hallo     If you choose to dynamically add attributes to a class instance  you certainly want to be able to undo it by writing  del a a

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There is a specific example of when you should use del  there may be others  but I know about this one off hand  when you are using sys exc info   to inspect an exception   This function returns a tuple  the type of exception that was raised  the message  and a traceback     The first two values are usually sufficient to diagnose an error and act on it  but the third contains the entire call stack between where the exception was raised and where the the exception is caught   In particular  if you do something like  try      do evil   except      exc type  exc value  tb   sys exc info       if something exc value           raise   the traceback  tb ends up in the locals of the call stack  creating a circular reference that cannot be garbage collected   Thus  it is important to do   try      do evil   except      exc type  exc value  tb   sys exc info       del tb     if something exc value           raise   to break the circular reference   In many cases where you would want to call sys exc info    like with metaclass magic  the traceback is useful  so you have to make sure that you clean it up before you can possibly leave the exception handler   If you don t need the traceback  you should delete it immediately  or just do   exc type  exc value   sys exc info    2    To avoid it all together

[python] When is del useful in Python?

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