I am curious about the details of __del__
in python, when and why it should be used and what it shouldn't be used for. I've learned the hard way that it is not really like what one would naively expected from a destructor, in that it is not the opposite of __new__
/ __init__
.
class Foo(object):
def __init__(self):
self.bar = None
def open(self):
if self.bar != 'open':
print 'opening the bar'
self.bar = 'open'
def close(self):
if self.bar != 'closed':
print 'closing the bar'
self.bar = 'close'
def __del__(self):
self.close()
if __name__ == '__main__':
foo = Foo()
foo.open()
del foo
import gc
gc.collect()
I saw in the documentation that it is not guaranteed __del__()
methods are called for objects that still exist when the interpreter exits.
Foo
instances existing when the interpreter exits, the bar is closed?del foo
or on gc.collect()
... or neither? if you want finer control of those details (e.g. the bar should be closed when the object is unreferenced) what is the usual way to implement that?__del__
is called is it guaranteed that __init__
has already been called? what about if the __init__
raised?The way to close resources are context managers, aka the with
statement:
class Foo(object):
def __init__(self):
self.bar = None
def __enter__(self):
if self.bar != 'open':
print 'opening the bar'
self.bar = 'open'
return self # this is bound to the `as` part
def close(self):
if self.bar != 'closed':
print 'closing the bar'
self.bar = 'close'
def __exit__(self, *err):
self.close()
if __name__ == '__main__':
with Foo() as foo:
print foo, foo.bar
output:
opening the bar
<__main__.Foo object at 0x17079d0> open
closing the bar
2) Python's objects get deleted when their reference count is 0. In your example the del foo
removes the last reference so __del__
is called instantly. The GC has no part in this.
class Foo(object):
def __del__(self):
print "deling", self
if __name__ == '__main__':
import gc
gc.disable() # no gc
f = Foo()
print "before"
del f # f gets deleted right away
print "after"
output:
before
deling <__main__.Foo object at 0xc49690>
after
The gc
has nothing to do with deleting your and most other objects. It's there to clean up when simple reference counting does not work, because of self-references or circular references:
class Foo(object):
def __init__(self, other=None):
# make a circular reference
self.link = other
if other is not None:
other.link = self
def __del__(self):
print "deling", self
if __name__ == '__main__':
import gc
gc.disable()
f = Foo(Foo())
print "before"
del f # nothing gets deleted here
print "after"
gc.collect()
print gc.garbage # The GC knows the two Foos are garbage, but won't delete
# them because they have a __del__ method
print "after gc"
# break up the cycle and delete the reference from gc.garbage
del gc.garbage[0].link, gc.garbage[:]
print "done"
output:
before
after
[<__main__.Foo object at 0x22ed8d0>, <__main__.Foo object at 0x22ed950>]
after gc
deling <__main__.Foo object at 0x22ed950>
deling <__main__.Foo object at 0x22ed8d0>
done
3) Lets see:
class Foo(object):
def __init__(self):
raise Exception
def __del__(self):
print "deling", self
if __name__ == '__main__':
f = Foo()
gives:
Traceback (most recent call last):
File "asd.py", line 10, in <module>
f = Foo()
File "asd.py", line 4, in __init__
raise Exception
Exception
deling <__main__.Foo object at 0xa3a910>
Objects are created with __new__
then passed to __init__
as self
. After a exception in __init__
, the object will typically not have a name (ie the f =
part isn't run) so their ref count is 0. This means that the object is deleted normally and __del__
is called.