Use self.symbol as consistent string representation

[tv3141]

Currently, the string representation uses self.origin - the string used to create the instance. After an arithmetic operation, the returned instance does not have this attribute any more and uses self.name, which is most likely different from self.origin. self.name also spells out the base units which is very verbose.

I suggest using self.symbol as string representation:

import cf_units

watt = cf_units.Unit('kg meter2 second-3')
print(watt)             # kg meter*meter/(second*second*second)
print(watt.symbol)      # W

another_watt = watt * 1
print(another_watt)            # watt -> changed
print(another_watt.symbol)     # W

watt_meter = watt * cf_units.Unit('meter')
print(watt_meter)            # meter^3-kilogram-second^-3  -> verbose
print(watt_meter.symbol)     # m3.kg.s-3

Before merging: Further changes in doc strings might be necessary.

[djkirkham]

Hi @tv3141, thanks for submitting this PR.

The origin string can give information about the units that is lost when using self.symbol. For example, the units for humidity are 'kg/kg' or 'g/kg', but using self.symbol would just give '1' or '0.001 1'.

Also, the origin string can be more understandable for non-SI units. When using self.symbol, 'miles hour-1' becomes '0.44704 m.s-1'.

I agree that self.name is verbose. Perhaps we could use self.origin or self.symbol instead.

[tv3141]

I see the value of self.originin special cases, but thought consistent general behavior, i.e. the unit string does not change, unless the unit changes, would outweigh this.

I think using self.origin or self.symbol would be an improvement. The string representation of a unit can then always be used for human consumption, such as in a plot.

[coveralls]

Coverage remained the same at 90.653% when pulling 05d1764 on tv3141:consistent_string_representation into bd988a4 on SciTools:master.

[coveralls]

Coverage remained the same at 90.653% when pulling 05d1764 on tv3141:consistent_string_representation into bd988a4 on SciTools:master.