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.. _systems:
Different Unit Systems (and default units)
==========================================
Pint Unit Registry has the concept of system, which is a group of units
>>> import pint
>>> ureg = pint.UnitRegistry(system='mks')
>>> ureg.default_system
'mks'
This has an effect in the base units. For example:
>>> q = 3600. * ureg.meter / ureg.hour
>>> q.to_base_units()
<Quantity(1.0, 'meter / second')>
But if you change to cgs:
>>> ureg.default_system = 'cgs'
>>> q.to_base_units()
<Quantity(100.0, 'centimeter / second')>
or more drastically to:
>>> ureg.default_system = 'imperial'
>>> '{:.3f}'.format(q.to_base_units())
'1.094 yard / second'
..warning: In versions previous to 0.7 `to_base_units` returns quantities in the
units of the definition files (which are called root units). For the definition file
bundled with pint this is meter/gram/second. To get back this behaviour use `to_root_units`,
set `ureg.system = None`
You can also use system to narrow down the list of compatible units:
>>> ureg.default_system = 'mks'
>>> ureg.get_compatible_units('meter')
frozenset({<Unit('light_year')>, <Unit('angstrom')>})
or for imperial units:
>>> ureg.default_system = 'imperial'
>>> ureg.get_compatible_units('meter')
frozenset({<Unit('thou')>, <Unit('league')>, <Unit('nautical_mile')>, <Unit('inch')>, <Unit('mile')>, <Unit('yard')>, <Unit('foot')>})
You can check which unit systems are available:
>>> dir(ureg.sys)
['US', 'cgs', 'imperial', 'mks']
Or which units are available within a particular system:
>>> dir(ureg.sys.imperial)
['UK_hundredweight', 'UK_ton', 'acre_foot', 'cubic_foot', 'cubic_inch', 'cubic_yard', 'drachm', 'foot', 'grain', 'imperial_barrel', 'imperial_bushel', 'imperial_cup', 'imperial_fluid_drachm', 'imperial_fluid_ounce', 'imperial_gallon', 'imperial_gill', 'imperial_peck', 'imperial_pint', 'imperial_quart', 'inch', 'long_hunderweight', 'long_ton', 'mile', 'ounce', 'pound', 'quarter', 'short_hunderdweight', 'short_ton', 'square_foot', 'square_inch', 'square_mile', 'square_yard', 'stone', 'yard']
Notice that this give you the opportunity to choose within units with colliding names:
>>> (1 * ureg.sys.imperial.pint).to('liter')
<Quantity(0.5682612500000002, 'liter')>
>>> (1 * ureg.sys.US.pint).to('liter')
<Quantity(0.47317647300000004, 'liter')>
>>> (1 * ureg.sys.US.pint).to(ureg.sys.imperial.pint)
<Quantity(0.8326741846289889, 'imperial_pint')>
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