Checking for dlls in ctypes

Wanderer

unread,

Oct 12, 2012, 11:36:27 AM10/12/12

to

I'm trying to write some code that will load one of three dll depending on the one available. I've tried the code below, but it doesn't work. The try except doesn't catch the exception. Is there a way to do this?

try:
self.dll = windll.pvcam64
except:
print "No pvcam64"
try:
self.dll = windll.pvcam32
except:
print "No pvcam32"
try:
self.dll = windll.pvcam
except:
print "No pvcam"
return
else:
print "installed pvcam"
else:
print "installed pvcam32"
else:
print "installed pvcam64"

Dave Angel

unread,

Oct 12, 2012, 12:28:17 PM10/12/12

to Wanderer, pytho...@python.org

I can't help you find the dll's, because I don't run Windows. But I
could help you write a clearer question:

"doesn't work" is thoroughly useless for describing errors. If you're
getting an exception, show us the full traceback. That will show which
statement got the exception that wasn't caught. Next question is which
of the dlls is missing. Are you getting an exception because it's
missing or because of something more fundamental, like nesting exception
handlers?

Using bare excepts is almost never a good idea. If it "works" you get
no clues what went wrong. For example, a typo in source code can
trigger a bare exception, as can a user typing Ctrl-C. So when you're
using bare excepts, you have robbed the user of any way to terminate the
program.

If I were you, I'd be writing a loop so there's only one try block. Too
much duplicated code in the way you're doing it.

--

DaveA

MRAB

unread,

Oct 12, 2012, 12:57:02 PM10/12/12

to pytho...@python.org

This works for me:

for name in ("pvcam64", "pvcam32", "pvcam"):
try:
self.dll = getattr(windll, name)
except OSError:
print "No " + name
else:
print "Installed " + name
return

Wanderer

unread,

Oct 12, 2012, 1:52:38 PM10/12/12

to pytho...@python.org

Yes that works for me, too. Thanks

Wanderer

unread,

Oct 12, 2012, 1:52:38 PM10/12/12

to comp.lan...@googlegroups.com, pytho...@python.org

On Friday, October 12, 2012 12:57:06 PM UTC-4, MRAB wrote:

Wanderer

unread,

Oct 12, 2012, 1:57:09 PM10/12/12

to Wanderer, pytho...@python.org, d...@davea.name

Sorry. It was a WindowsError, but the code I posted now works for me and I can't reproduce the problem. I'll be more diligent in the future.

Wanderer

unread,

Oct 12, 2012, 1:57:09 PM10/12/12

to comp.lan...@googlegroups.com, pytho...@python.org, d...@davea.name, Wanderer

On Friday, October 12, 2012 12:29:02 PM UTC-4, Dave Angel wrote:

Nobody

unread,

Oct 13, 2012, 7:32:14 AM10/13/12

to

On Fri, 12 Oct 2012 12:28:17 -0400, Dave Angel wrote:

> Using bare excepts is almost never a good idea. If it "works" you get no
> clues what went wrong. For example, a typo in source code can trigger a
> bare exception, as can a user typing Ctrl-C. So when you're using bare
> excepts, you have robbed the user of any way to terminate the program.

If you want to catch any error, use "except StandardError:". That covers
all errors but not things like KeyboardInterrupt (Ctrl-C) or SystemExit
(sys.exit()).

In situations such as this, where you try multiple candidates until one
succeeds, there's no reason to be any more specific than that. In any
case, Python's lack of formal interfaces makes it hard to reliably be more
specific.

However: you should bear in mind that loading the wrong DLL may just
result in an OS-level exception (e.g. segfault), which can't be caught.
It's preferable to allow the DLL to be explicitly selected e.g. in a
configuration file.

88888 Dihedral

unread,

Oct 13, 2012, 9:40:01 AM10/13/12

to

Wanderer於 2012年10月12日星期五UTC+8下午11時36分27秒寫道：

In linux there are shared libraries that could be linked in the runtime.

But for the security concerns this requres the system administrator account
to install shared libraries.