Checking for dlls in ctypes

Discussion in 'Python' started by Wanderer, Oct 12, 2012.

  1. Wanderer

    Wanderer Guest

    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"
     
    Wanderer, Oct 12, 2012
    #1
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  2. Wanderer

    Dave Angel Guest

    On 10/12/2012 11:36 AM, Wanderer wrote:
    > 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"
    >


    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
     
    Dave Angel, Oct 12, 2012
    #2
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  3. Wanderer

    MRAB Guest

    On 2012-10-12 16:36, Wanderer wrote:
    > 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"
    >

    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
     
    MRAB, Oct 12, 2012
    #3
  4. Wanderer

    Wanderer Guest

    On Friday, October 12, 2012 12:57:06 PM UTC-4, MRAB wrote:
    > On 2012-10-12 16:36, Wanderer wrote:
    >
    > > 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"

    >
    > >

    >
    > 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


    Yes that works for me, too. Thanks
     
    Wanderer, Oct 12, 2012
    #4
  5. Wanderer

    Wanderer Guest

    On Friday, October 12, 2012 12:57:06 PM UTC-4, MRAB wrote:
    > On 2012-10-12 16:36, Wanderer wrote:
    >
    > > 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"

    >
    > >

    >
    > 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


    Yes that works for me, too. Thanks
     
    Wanderer, Oct 12, 2012
    #5
  6. Wanderer

    Wanderer Guest

    On Friday, October 12, 2012 12:29:02 PM UTC-4, Dave Angel wrote:
    > On 10/12/2012 11:36 AM, Wanderer wrote:
    >
    > > 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"

    >
    > >

    >
    >
    >
    > 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


    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, Oct 12, 2012
    #6
  7. Wanderer

    Wanderer Guest

    On Friday, October 12, 2012 12:29:02 PM UTC-4, Dave Angel wrote:
    > On 10/12/2012 11:36 AM, Wanderer wrote:
    >
    > > 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"

    >
    > >

    >
    >
    >
    > 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


    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, Oct 12, 2012
    #7
  8. Wanderer

    Nobody Guest

    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.
     
    Nobody, Oct 13, 2012
    #8
  9. Wandereræ–¼ 2012å¹´10月12日星期五UTC+8下åˆ11時36分27秒寫é“:
    > 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"


    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.
     
    88888 Dihedral, Oct 13, 2012
    #9
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