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.