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?

Asked By: wilhelmtell

||

Answers:

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.

Answered By: Unknown

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

Answered By: Bastien Léonard

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
Answered By: Clint Miller

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).

Answered By: Virgil Dupras

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

Answered By: n3o59hf

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
Answered By: Tobias Kienzler

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
>>> from package import *
>>> p = Package()
>>> q = Package()
>>> q.files = ['a', 'b', 'c']
>>> quit()
Running cleanup...
Unlinking file: a
Unlinking file: b
Unlinking file: c
Running cleanup...
Answered By: ostrokach

A better alternative is to use weakref.finalize. See the examples at Finalizer Objects and Comparing finalizers with __del__() methods.

Answered By: SCGH

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 <module>                                                                                                                                                          
    fixture.method()                                                                                                                                                                                              
AttributeError: 'NoneType' object has no attribute 'method'

Hope it helps the next person.

Answered By: user2394284

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.

Answered By: ViFI

The absolute best way is to combine both approaches.

To implement a context manager for explicit life-cycle handling. As well as handle cleanup in case the user forgets it or it is not convenient to use a with statement. This is best done by weakref.finalize.

import os
from typing import List
import weakref

class Package:
    def __init__(self):
        self.files = []
        self._finalizer = weakref.finalize(self, self._cleanup_files, self.files)

    @staticmethod
    def _cleanup_files(files: List):
        for file in files:
            os.unlink(file)

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self._finalizer()

"weakref.finalize returns a callable finalizer object which will be called when obj is garbage collected. Unlike an ordinary weak reference, a finalizer will always survive until the reference object is collected, greatly simplifying lifecycle management."

Unlike atexit.register the object is not held in memory until the interpreter is shut down.

And unlike object.__del__, weakref.finalize is guaranteed to be called at interpreter shutdown. So it is much more safe.

Answered By: Chris
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