Contexts: a new feature to simplify common conversions in specialized situations

[hgrecco]

`If you work in certain topics, you usually need to convert from certain units to others based some pre-established relationships.

For example, you are working with EM fields in vacuum and you need to transform wavelength to frequency. If you try this:

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

You need to use the relation wavelength = speed_of_light * frequency.

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

The plan is to provide a context (abbreviated ctx) to facilitate this:

>>> q.to('Hz', ctx='m = c * s')
<Quantity(5.99584916e+14, 'hertz')>

or alternative as a context manager:

>>> with ureg.ctx('m = c * s'):
>>>     q.to('Hz')
<Quantity(5.99584916e+14, 'hertz')>

Some context will be predefined in the definitions file. The proposed syntax is:

@context spectroscopy = sp
    m = c * s

which then can be used as:

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

or with the abbreviated form:

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

You can define multiple relations in a single context (one per line in the definition file or in an tuple when you define it in situ)

Open points:

• naming: Alternative names for ctx are: context, using
• definitions: Maybe is clearer to define contexts with dimensions. eg: [length] = c * [time]

[cbrunet]

I hope you will define the relation as wavelength = speed_of_light / frequency instead... ;-)

It could be useful also to be able to parametrize the context. For example, the speed of light depends on the refractive index of the material. For such situation, I propose something like:

@context(n = 1) spectroscopy = sp
    m = c / n / Hz

then we could use it like

q.to('Hz', ctx='spectroscopy', n=1.444)

and towould be defined like

def to(self, other=None, ctx=None, **kwargs):

Off course, parameters would be optionals, and have default values.

It would also be useful to have a way to define a default context. For example:

ur = UnitRegistry() 
ur.context('spectroscopy', n=1.444)

[hgrecco]

I cannot really define wavelength = speed_of_light / frequency because pint is about units and dimensions. It knows about meter and second, about [length], [time], [frequency] but not about wavelength. You measure wavelengths in meters which and they have length dimension.

I understand that as a user this might not be the most straight forward way to declare. m = c * s looks strange. Maybe [length] = c * [time]. I am open to ideas.

I really like the idea of parametrization of the context. We just need find a (pythonic) way to indicate that n is a parameter, no something to take from the registry.

I also like the idea of a default context. I am actually looking into the idea of setting several defaults including format and base units. Context should be included as well.

[cbrunet]

Another proposition for definition of context could be something like:

@context spectroscopy = sp:
    @parameter n = []; default: 1 # unitless, with default value
    speed_of_light = 299792458 / n * meter / second = c # redefinition of a constant inside the context
    meter = c / hertz # wavelength <-> frequency

[hgrecco]

I think I like your original proposal of parametrization:

@context(n = 1) spectroscopy = sp
    m = c / n / Hz

and:

q.to('Hz', ctx='spectroscopy', n=1.444)

Let's try this and see how it works.

[hgrecco]

Implemented Context Parsing 4aba2db47707cffd4c7f49cfe0d423f32f16de4c