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 check which unit systems are available:

>>> dir(ureg.sys)
['Planck', 'SI', 'US', 'atomic', 'cgs', 'imperial', 'mks']

Or which units are available within a particular system:

>>> dir(ureg.sys.imperial)
['UK_force_ton', 'UK_hundredweight', ... 'cubic_foot', 'cubic_inch', ... 'thou', 'ton', 'yard']

Notice that this give you the opportunity to choose within units with colliding names:

>>> (1 * ureg.sys.imperial.pint).to('liter')
<Quantity(0.568261..., 'liter')>
>>> (1 * ureg.sys.US.pint).to('liter')
<Quantity(0.473176..., 'liter')>
>>> (1 * ureg.sys.US.pint).to(ureg.sys.imperial.pint)
<Quantity(0.832674..., 'imperial_pint')>