cube.convert_units converting from 'unknown'

[rcomer]

I just noticed that cube.convert_units works when the cube doesn't have an existing unit. E.g.:

mycube = iris.cube.Cube(1.)
mycube.convert_units('mm day-1')
print mycube
print mycube.data

produces this output:

unknown / (mm day-1) 
1.0

Shouldn't convert_units produce an exception if it doesn't have a unit to map from?

[ajdawson]

I agree. The code for convert_units looks like this:

def convert_units(self, unit):
    # If the cube has units convert the data.
    if not self.units.is_unknown():
        self.data = self.units.convert(self.data, unit)
    self.units = unit

So we are happily changing the unit to the new unit without manipulating data values. In my mind this corresponds to just setting the units attribute. Therefore, we should raise an exception when converting units from 'unknown' and just instruct the user to set the units attribute directly, since this operation is directly equivalent to the current (surprising) behaviour of convert_units:

def convert_units(self, unit):
    if self.units.is_unknown():
        raise SomeRelevantException('cannot convert from unknown units to {!s}, '
                                    'set the "units" attribute instead'.format(unit))
    self.data = self.units.convert(self.data, unit)
    self.units = unit

[rcomer]

Fixed at #2700.