Handling functions that return non-zero gracefully #11

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1. clade commented on Apr 9, 2014

   

   clade

   on Apr 9, 2014

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   Hi,

   I didn't know this feature. It should be possible to change the way
   error are caught for this function. I post a solution on a new branch
   (DAQmxGetSysFunction). Can you test it ? (I havn't)

   Pierre

   Le 09/04/2014 09:03, Sidney Cadot a écrit :

   Hi Pierre,

   I've been toying with PyDAQmx for a bit, thanks for this -- looks like
   a well-done wrapper.

   One thing I bumped into was this:

   In C, it is possible to use PyDAQmx.DAQmxGetSysDevNames() with a NULL
   char* argument to get the number of characters that need to be
   reserved for the characater buffer. This will return a positive value
   that does not indicate a warning.

   However, PyDAQmx wraps the C functions inside the 'mafunction' wrapper
   that auto-throws an exception whenever a function returns non-zero. So
   it is impossible (or at least a bit cumbersome) to use this function
   in this way.

   Is there an easy way around this?

   The best I could think of now was to catch the exception and extract
   the "error", by hand, like this, which seems rather roundabout. In the
   process, I also need to silence stdout because mafunction() prints a
   WARNING message when warnings happen.

   |def GetSysDevNames():

    import sys, io
   
    # We know that PyDAQmx.DAQmxGetSysDevNames(None, 0)
    # will return a positive value; PyDAQmx intercepts
    # this, prints a warning, then throws a DAQError in respose.
    #
    # To work around this, we need to
    #
    #   (1) silence the print()
    #   (2) catch the error
   
    old_stdout = sys.stdout
    sys.stdout = io.StringIO() # dummy fiel to catch print() output
   
    try:
        numBytesNeeded = PyDAQmx.DAQmxGetSysDevNames(None, 0)
    except PyDAQmx.DAQError as e:
        numBytesNeeded = e.error
   
    # Restore stdout
    sys.stdout = old_stdout
   
    # We now know how many bytes are needed.
    # Allocate the buffer
   
    stringBuffer = ctypes.create_string_buffer(numBytesNeeded)
   
    # Get the device names
    PyDAQmx.DAQmxGetSysDevNames(stringBuffer, numBytesNeeded)
   
    # Extract the device name string
    return ctypes.string_at(stringBuffer)
   

   |
   |

   Is there a simpler/better way to do this? It could be useful to
   provide a way to call NIDAQmx functions without invoking the automatic
   translation of nonzero return values to DAQError exceptions.

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   #11.
2. sidneycadot commented on Apr 10, 2014

   

   sidneycadot

   on Apr 10, 2014

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   I checked out that branch.

   What is the proper way of testing it -- should I just do a "python setup.py install" (does that work properly on Python 3.3) ? Or is it possible to test this locally inside that directory?

   Unfortunately, I do not fully understand the way Python handles modules.
3. apteryks commented on Sep 22, 2015

   

   apteryks

   on Sep 22, 2015

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   This seems to be fixed as of PyDAQmx 1.3.