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# Using Pint in your projects

## Having a shared registry

If you use Pint in multiple modules within your Python package, you normally want to avoid creating multiple instances of the unit registry. The best way to do this is by instantiating the registry in a single place. For example, you can add the following code to your package `__init__.py`

    >>> from pint import UnitRegistry
    >>> ureg = UnitRegistry()
    >>> Q_ = ureg.Quantity

Then in `yourmodule.py` the code would be

    from . import ureg, Q_

    length = 10 * ureg.meter
    my_speed = Q_(20, 'm/s')

If you are pickling and unpickling Quantities within your project, you should also define the registry as the application registry

    from pint import UnitRegistry, set_application_registry
    ureg = UnitRegistry()
    set_application_registry(ureg)

Warning

There are no global units in Pint. All units belong to a registry and you can have multiple registries instantiated at the same time. However, you are not supposed to operate between quantities that belong to different registries. Never do things like this:

    >>> q1 = 10 * UnitRegistry().meter
    >>> q2 = 10 * UnitRegistry().meter
    >>> q1 + q2
    Traceback (most recent call last):
    ...
    ValueError: Cannot operate with Quantity and Quantity of different registries.
    >>> id(q1._REGISTRY) == id(q2._REGISTRY)
    False

## Keeping up to date with Pint development

While we work hard to avoid breaking code using Pint, sometimes it happens. To help you track how Pint is evolving it is recommended that you run a daily or weekly job against pint master branch.

For example, this is how xarray is doing it:

If a new version of Pint breaks your code, please open an issue to let us know.