Question

Can someone explain why the following code behaves the way it does:

import types

class Dummy():
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print "delete",self.name

d1 = Dummy("d1")
del d1
d1 = None
print "after d1"

d2 = Dummy("d2")
def func(self):
    print "func called"
d2.func = types.MethodType(func, d2)
d2.func()
del d2
d2 = None
print "after d2"

d3 = Dummy("d3")
def func(self):
    print "func called"
d3.func = types.MethodType(func, d3)
d3.func()
d3.func = None
del d3
d3 = None
print "after d3"

The output (note that the destructor for d2 is never called) is this (python 2.7)

delete d1
after d1
func called
after d2
func called
delete d3
after d3

Is there a way to “fix” the code so the destructor is called without deleting the method added? I mean, the best place to put the d2.func = None would be in the destructor!

Thanks

[edit] Based on the first few answers, I’d like to clarify that I’m not asking about the merits (or lack thereof) of using __del__. I tried to create the shortest function that would demonstrate what I consider to be non-intuitive behavior. I’m assuming a circular reference has been created, but I’m not sure why. If possible, I’d like to know how to avoid the circular reference….

Answer 1

I’m providing my own answer because, while I appreciate the advice to avoid __del__, my question was how to get it to work properly for the code sample provided.

Short version: The following code uses weakref to avoid the circular reference. I thought I’d tried this before posting the question, but I guess I must have done something wrong.

import types, weakref

class Dummy():
    def __init__(self, name):
        self.name = name
    def __del__(self):
        print "delete",self.name

d2 = Dummy("d2")
def func(self):
    print "func called"
d2.func = types.MethodType(func, weakref.ref(d2)) #This works
#d2.func = func.__get__(weakref.ref(d2), Dummy) #This works too
d2.func()
del d2
d2 = None
print "after d2"

Longer version:
When I posted the question, I did search for similar questions. I know you can use with instead, and that the prevailing sentiment is that __del__ is BAD.

Using with makes sense, but only in certain situations. Opening a file, reading it, and closing it is a good example where with is a perfectly good solution. You’ve gone a specific block of code where the object is needed, and you want to clean up the object and the end of the block.

A database connection seems to be used often as an example that doesn’t work well using with, since you usually need to leave the section of code that creates the connection and have the connection closed in a more event-driven (rather than sequential) timeframe.

If with is not the right solution, I see two alternatives:

1. You make sure __del__ works (see this blog for a better
   description of weakref usage)
2. You use the atexit module to run a callback when your program closes. See this topic for example.

While I tried to provide simplified code, my real problem is more event-driven, so with is not an appropriate solution (with is fine for the simplified code). I also wanted to avoid atexit, as my program can be long-running, and I want to be able to perform the cleanup as soon as possible.

So, in this specific case, I find it to be the best solution to use weakref and prevent circular references that would prevent __del__ from working.

This may be an exception to the rule, but there are use-cases where using weakref and __del__ is the right implementation, IMHO.