For arguments given in wrong order, error message could be more helpful

[aawdls]

I made a mistake in an element.get_value call, where I got the order of the arguments wrong. Pytac accepted my units constant as a choice of handle and gave the corresponding error. It would be nicer if it could notice my mistake and give a more useful message.

corrector = lattice.get_elements("HSTR")[5]
corrector.get_value("x_kick", pytac.ENG)



---------------------------------------------------------------------------
HandleException                           Traceback (most recent call last)
<ipython-input-20-2ba9c33239e9> in <module>()
      1 # Get a corrector magnet
      2 corrector = lattice.get_elements("HSTR")[5]
----> 3 corrector.get_value("x_kick", pytac.ENG)
      4 

/scratch/atip/pytac/pytac/element.pyc in get_value(self, field, handle, units, data_source, throw)
    221         try:
    222             return self._data_source_manager.get_value(field, handle, units,
--> 223                                                        data_source, throw)
    224         except DataSourceException:
    225             raise DataSourceException("No data source {0} on element {1}."

/scratch/atip/pytac/pytac/data_source.pyc in get_value(self, field, handle, units, data_source, throw)
    211         try:
    212             data_source = self._data_sources[data_source]
--> 213             value = data_source.get_value(field, handle, throw)
    214             return self._uc[field].convert(value, origin=data_source.units,
    215                                            target=units)

/scratch/atip/pytac/pytac/data_source.pyc in get_value(self, field, handle, throw)
    335         """
    336         try:
--> 337             return self._devices[field].get_value(handle, throw)
    338         except KeyError:
    339             raise FieldException("No field {0} on data source {1}."

/scratch/atip/pytac/pytac/device.pyc in get_value(self, handle, throw)
    177         else:
    178             raise HandleException("Device {0} has no {1} PV.".format(self.name,
--> 179                                                                      handle))
    180 
    181     def set_value(self, value, throw=True):

HandleException: Device SR01A-PC-HSTR-06 has no engineering PV.