# Contexts¶

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 you do this directly:

>>> import pint
>>> ureg = pint.UnitRegistry()
>>> q = 500 * ureg.nm
>>> q.to('Hz')
Traceback (most recent call last):
...
DimensionalityError: Cannot convert from 'nanometer' ([length]) to 'hertz' (1 / [time])


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.

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


or with the abbreviated form:

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


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

>>> with ureg.context('sp'):
...     q.to('Hz')
<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:

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


This works also using the with statement:

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


or in the registry:

>>> ureg.enable_contexts('sp', 'boltzmann')
>>> q.to('Hz')
<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') :
...         q.to('Hz')
<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 = wl.to('Hz', 'sp')
>>> f.to('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')
>>> substance.to('moles', 'chemistry', mw = 5 * ureg('g/mol'))
<Quantity(19.0, 'mole')>


## Ensuring context when calling a function¶

Pint provides a decorator to make sure that a function called is done within a given context. Just like before, you have to provide as argument the name (or alias) of the context and the parameters that you wish to set.

>>> wl = 530. * ureg.nm
>>> @ureg.with_context('sp', n=1.33)
... def f(wl):
...     return wl.to('Hz').magnitude
>>> f(wl)
398.496240602


This decorator can be combined with wraps or check decorators described in Wrapping and checking functions.

## 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
@end


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')
...                      lambda ureg, x: x / ureg.speed_of_light)
...                      lambda ureg, x: x * ureg.speed_of_light)
>>> ureg("1 s").to("km", "ab")
299792.458 kilometer


It is also possible to create anonymous contexts without invoking add_context:

>>> c = pint.Context()
...
>>> ureg("1 s").to("km", c)
299792.458 kilometer


## Using contexts for unit redefinition¶

The exact definition of a unit of measure can change slightly depending on the country, year, and more in general convention. For example, the ISO board released over the years several revisions of its whitepapers, which subtly change the value of some of the more obscure units. And as soon as one steps out of the SI system and starts wandering into imperial and colonial measuring systems, the same unit may start being defined slightly differently every time - with no clear ‘right’ or ‘wrong’ definition.

The default pint definitions file (default_en.txt) tries to mitigate the problem by offering multiple variants of the same unit by calling them with different names; for example, one will find multiple definitions of a “BTU”:

british_thermal_unit = 1055.056 * joule = Btu = BTU = Btu_iso
international_british_thermal_unit = 1e3 * pound / kilogram * degR / kelvin * international_calorie = Btu_it
thermochemical_british_thermal_unit = 1e3 * pound / kilogram * degR / kelvin * calorie = Btu_th


That’s sometimes insufficient, as Wikipedia reports no less than 6 different definitions for BTU, and it’s entirely possible that some companies in the energy sector, or even individual energy contracts, may redefine it to something new entirely, e.g. with a different rounding.

Pint allows changing the definition of a unit within the scope of a context. This allows layering; in the example above, a company may use the global definition of BTU from default_en.txt above, then override it with a customer-specific one in a context, and then override it again with a contract-specific one on top of it.

A redefinition follows the following syntax:

<unit name> = <new definition>


where <unit name> can be the base unit name or one of its aliases. For example:

BTU = 1055 J


Programmatically:

>>> ureg = pint.UnitRegistry()
>>> q = ureg.Quantity("1 BTU")
>>> q.to("J")
1055.056 joule
>>> ctx = pint.Context()
>>> ctx.redefine("BTU = 1055 J")
>>> q.to("J", ctx)
1055.0 joule
# When the context is disabled, pint reverts to the base definition
>>> q.to("J")
1055.056 joule


Or with a definitions file:

@context somecontract
BTU = 1055 J
@end

>>> ureg = pint.UnitRegistry()
>>> q = ureg.Quantity("1 BTU")
>>> q.to("J")
1055.056 joule
>>> q.to("J", "somecontract")
1055.0 joule


Note

Redefinitions are transitive; if the registry defines B as a function of A and C as a function of B, redefining B will also impact the conversion from C to A.

Limitations

• You can’t create brand new units ; all units must be defined outside of the context first.
• You can’t change the dimensionality of a unit within a context. For example, you can’t define a context that redefines grams as a force instead of a mass (but see the unit force_gram in default_en.txt).
• You can’t redefine a unit with a prefix; e.g. you can redefine a liter, but not a decaliter.
• You can’t redefine a base unit, such as grams.
• You can’t add or remove aliases, or change the symbol. Symbol and aliases are automatically inherited from the UnitRegistry.
• You can’t redefine dimensions or prefixes.

## Working without a default definition¶

In some cases, the definition of a certain unit may be so volatile to make it unwise to define a default conversion rate in the UnitRegistry.

This can be solved by using ‘NaN’ (any capitalization) instead of a conversion rate rate in the UnitRegistry, and then override it in contexts:

truckload = nan kg

@context Euro_TIR
@end

@context British_grocer
@end


This allows you, before any context is activated, to define quantities and perform dimensional analysis:

>>> ureg.truckload.dimensionality
[mass]