Problem with Lock.aquire() and user defined exceptions

[djw]

Greetings,

I am having some difficulty with some _single_ threaded code that
happens to contain thread locking around some data structure access.
There is locking code beacuse I am going to have another thread access
the data in the next rev of the code.

Red Hat Linux 9, Python 2.2.1


import thread

devices = None
device_lock = None

def getDevice(d):
global devices, device_lock
val = None
print device_lock.locked() # prints 0
device_lock.acquire() # hangs (blocks)
try:
val = devices[d]
finally:
device_lock.release()
return val

def init():
global devices, device_lock
devices = {}
device_lock = thread.allocate_lock()

init()
devices["foo"] = 1 # not the way the program really operates, but you
get the idea
x = getDevice("foo") # prints 0, then hangs

This code is a bit of a contrived excerpt, but you get the idea. When I
run this code in the framework of my larger program, the program prints
0 and then hangs on the acquire() call.

I might point out that this code is being called from a handler that is
being called from asyncore, so there are several levels above this code
(at least above getDevice()).

I am having a strangely similar, but different issue when I try to raise
user defined exceptions in my code. If I define:

class DeviceError(Exception):
pass

and then:

raise DeviceError

somewhere in my code, my program hangs, requiring a kill from the shell.

Is there some fundamental thing I am doing wrong here? This is the first
time I have ever gotten such unexpected results from a Python program
and I'm at a loss.

Thanks,

Don

 

[djw]

I was able to figure out both these problems. They both boiled down to a
try: finally: block at a higher level. Any sort of exception that occured
in code called from this block was getting "swallowed up"... is there a way
to determine in a try: finally: what exception (if any) occured? This seems
like a really dangerous construct, kind of like a "naked" try: except: that
doesn't specify exactly what exceptions are to be caught.

-Don