If you work frequently on certain topics, you will probably find the need to convert between dimensions based on some pre-established (physical) relationships. For example, in spectroscopy you need to transform from wavelength to frequency. These are incompatible units and therefore Pint will raise an error if your do this directly:

>>> import pint
>>> ureg = pint.UnitRegistry()
>>> q = 500 * ureg.nm
Traceback (most recent call last):
pint.unit.DimensionalityError: Cannot convert

You probably want to use the relation frequency = speed_of_light / wavelength:

>>> (ureg.speed_of_light / q).to('Hz')
<Quantity(5.99584916e+14, 'hertz')>

To make this task easy, Pint has the concept of contexts which provides conversion rules between dimensions. For example, the relation between wavelength and frequency is defined in the spectroscopy context (abbreviated sp). You can tell pint to use this context when you convert a quantity to different units.

>>>'Hz', 'spectroscopy')
<Quantity(5.99584916e+14, 'hertz')>

or with the abbreviated form:

>>>'Hz', 'sp')
<Quantity(5.99584916e+14, 'hertz')>

Contexts can be also enabled for blocks of code using the with statement:

>>> with ureg.context('sp'):
<Quantity(5.99584916e+14, 'hertz')>

If you need a particular context in all your code, you can enable it for all operations with the registry:

>>> ureg.enable_contexts('sp')

To disable the context, just call:

>>> ureg.disable_contexts()

Enabling multiple contexts

You can enable multiple contexts:

>>>'Hz', 'sp', 'boltzmann')
<Quantity(5.99584916e+14, 'hertz')>

This works also using the with statement:

>>> with ureg.context('sp', 'boltzmann'):
<Quantity(5.99584916e+14, 'hertz')>

or in the registry:

>>> ureg.enable_contexts('sp', 'boltzmann')
<Quantity(5.99584916e+14, 'hertz')>

If a conversion rule between two dimensions appears in more than one context, the one in the last context has precedence. This is easy to remember if you think that the previous syntax is equivalent to nest contexts:

>>> with ureg.context('sp'):
...     with ureg.context('boltzmann') :
<Quantity(5.99584916e+14, 'hertz')>

Parameterized contexts

Contexts can also take named parameters. For example, in the spectroscopy you can specify the index of refraction of the medium (n). In this way you can calculate, for example, the wavelength in water of a laser which on air is 530 nm.

>>> wl = 530. * ureg.nm
>>> f ='Hz', 'sp')
>>>'nm', 'sp', n=1.33)
<Quantity(398.496240602, 'nanometer')>

Contexts can also accept Pint Quantity objects as parameters. For example, the ‘chemistry’ context accepts the molecular weight of a substance (as a Quantity with dimensions of [mass]/[substance]) to allow conversion between moles and mass.

>>> substance = 95 * ureg('g')
>>> moles ='moles', 'chemistry', mw = 5 * ureg('g/mol'))
<Quantity(19.0, 'mole')>

Defining contexts in a file

Like all units and dimensions in Pint, contexts are defined using an easy to read text syntax. For example, the definition of the spectroscopy context is:

@context(n=1) spectroscopy = sp
    # n index of refraction of the medium.
    [length] <-> [frequency]: speed_of_light / n / value
    [frequency] -> [energy]: planck_constant * value
    [energy] -> [frequency]: value / planck_constant

The @context directive indicates the beginning of the transformations which are finished by the @end statement. You can optionally specify parameters for the context in parenthesis. All parameters are named and default values are mandatory. Multiple parameters are separated by commas (like in a python function definition). Finally, you provide the name of the context (e.g. spectroscopy) and, optionally, a short version of the name (e.g. sp) separated by an equal sign. See the definition of the ‘chemistry’ context in default_en.txt for an example of a multiple-parameter context.

Conversions rules are specified by providing source and destination dimensions separated using a colon (:) from the equation. A special variable named value will be replaced by the source quantity. Other names will be looked first in the context arguments and then in registry.

A single forward arrow (->) indicates that the equations is used to transform from the first dimension to the second one. A double arrow (<->) is used to indicate that the transformation operates both ways.

Context definitions are stored and imported exactly like custom units definition file (and can be included in the same file as unit definitions). See “Defining units” for details.

Defining contexts programmatically

You can create Context object, and populate the conversion rules using python functions. For example:

>>> ureg = pint.UnitRegistry()
>>> c = pint.Context('ab')
>>> c.add_transformation('[length]', '[time]',
...                      lambda ureg, x: ureg.speed_of_light / x)
>>> c.add_transformation('[time]', '[length]',
...                      lambda ureg, x: ureg.speed_of_light * x)
>>> ureg.add_context(c)