hgrecco / pint / 377

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Pull #231

travis-ci

Introduce 'u' to format only a Quantity's units

It is currently not possible to correctly format units (UnitsContainer
objects) using the '~' format spec.

    >>> format(q, '~')
    '17.0 km / s ** 2'
    >>> format(q.units, '~')
    'kilometer / second ** 2'

Notice how the units are different when formatted on their own.

In order for units to be properly formatted with '~', the UnitsRegistry
must be available so that the long names in the units container can be
mapped to their short, symbolic names. I.e. 'kilo' -> 'k' and 'meter' ->
'm'. A key problem is that UnitsContainer objects do not have access to
the UnitRegistry. When quantities are formatted, we cheat (a little) and
use the quantity's UnitRegistry (_REGISTRY) to perform the _get_symbol()
mapping in the case of '~'.

So we introduce 'u' as a formatting spec for Quantity that causes only
the unit, and not the magnitude, to be returned from format().

    >>> format(q, '~u')
    'km / s ** 2'
    >>> format(q, '~Pu')
    'km/s²'

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