On Sun, 27 May 2007 23:20:49 -0700, Jack wrote:
> Thanks Steven, for the reply. Very helpful. I've got a lot to learn in
> Python :)
>
> Some questions:
>
>> (1) Python can automatically free most data structures and close open
>> files, but if your needs are more sophisticated, this approach may not be
>> suitable.
>
> Since it's a wrapper of a DLL or .so file, I actually need to call
> mylib_exit()
> to do whatever cleanup the library needs to do.
Well, there may be better ways, but I'd do something like this:
# === mylib module ===
# Exception raised if library is not initiated
class Uninitiated(Exception):
pass
def _init():
"""Private set-up code."""
global DATA, _INITIALIZED
DATA = range(100000) # or something more useful
_INITIALIZED = True
def _close():
"""Private tear-down code."""
global DATA, _INITIALIZED
del DATA, _INITIALIZED
def isinited():
"""Return True if the library is initialized, otherwise False."""
try:
_INITIALIZED; return True
except NameError:
return False
def init():
"""Public set-up code."""
if not isinited(): _init()
def close():
"""Public tear-down code."""
if isinited(): _close()
exit = close # alias for exit/close.
def func():
if isinited():
return DATA[0]
else:
raise Uninitiated("Library is not initialized")
All of the above can be modified to be in a class instead of a module.
That's especially useful if you can have multiple instances, perhaps with
different state.
>>> 2. what's the right way to call mylib_exit()? I put it in __del__(self)
>>> but it is not being called in my simple test.
>>
>> instance.__del__ is only called when there are no references to the
>> instance.
>
> I didn't call del explicitly. I'm expecting Python to call it when
> the program exits. I put a logging line in __del__() but I never
> see that line printed. It seems that __del__() is not being called
> even when the program exits. Any idea why?
Python tries really really hard to call instance.__del__() but there are
pathological cases where it just can't. Maybe you've found one of them.
But I'm guessing that you may have defined a __del__ method, but not
actually created an instance. If so, __del__ will never be called.
This should work:
class Foo(object):
def __del__(self):
print "All gone now"
instance = Foo()
When you exit, instance will be garbage collected and instance.__del__()
will be called. But without the instance, __del__ is not called on the
class directly.
Hope this was some help,
--
Steven.
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