Defining your own dimensionless units

[truemerrill]

First off, I am a big fan of Pint and am using it in several of my projects. I have a question about defining custom dimensionless units. As an example of what I am trying to do, consider how Pint behaves with 'pi':

import pint

Registry = pint.UnitRegistry()
Quantity = Registry.Quantity
print(Quantity(1, 'pi').to('dimensionless'))

This code snippet prints 3.14159265359 dimensionless. Now suppose I want to define a new constant representing the electron g-factor (or pick any other dimensionless physical constant). I would suppose such a unit could be defined as follows:

import pint

Registry = pint.UnitRegistry()
Registry.define('electron_g_factor = -2.00231930436 * dimensionless = gs')
Quantity = Registry.Quantity

# Create a quantity with units of g_factors
g = Quantity(1, 'gs')
print(g)

# Convert g to dimenionless, similar to 'pi' (RAISES KeyError)
print(g.to('dimensionless'))

The first print statement prints 1 electron_g_factor. I would expect the second statement to print -2.00231930436 dimensionless but instead it raises a KeyError. The second statement fails because Pint can't convert to a dimensionless quantity. What gives? How can I define a dimensionless unit that behaves like 'pi'?

[truemerrill]

I am using Pint 0.4.2 if that helps any.

[hgrecco]

Thanks for the comments and the support. It is great to see that people are using Pint.

The plan is to make it intuitive (as all other things in Pint!) so that you can do:

Registry.define('electron_g_factor = -2.00231930436 = gs')

(Notice that dimensionless is not in the definition, but it should not matter if it is there).

However, this does not work in 0.5 (the current version) nor in the older ones.

This is something that I hope to get fixed by 0.6, but we are still brainstorming how to improve the way constants (with and without units) are handled. See #159, #17

Right now, the only way to achieve this is using some of the private API (I have tested against 0.5, but it should work against 0.4.2)

from pint.unit import UnitDefinition, ScaleConverter
Registry.define(UnitDefinition('electron_g_factor', 'gs', (), ScaleConverter(-2.00231930436)))

[truemerrill]

Yep, I can confirm your method works in 0.4.2. Thanks for developing this extremely useful module, I can't tell you how many errors I have caught since using Pint in my physics codes.

[hgrecco]

Thanks again! I am happy that you find it useful and feel free to provide feedback in terms of features that could be added. I will try to provide a better way to add constants in 0.6, but the way I have showed you will continue to work.

[hgrecco]

I am closing this now. Feel free to reopen.

[lafrech]

I'm discovering pint and I came across this while trying to define percent and permille units.

The import paths have changed since @hgrecco's comment above. Here's an updated version.

from pint.definitions import UnitDefinition
from pint.converters import ScaleConverter

ureg.define(UnitDefinition('percent', None, (), ScaleConverter(1/100)))
ureg.define(UnitDefinition('permille', None, (), ScaleConverter(1/1000)))

val = 12 * ureg.percent
print(val.to('permille'))
# 120.0 permille
print(val.to('dimensionless'))
# 0.12 dimensionless

Unfortunately, we still can't use the string form definition:

ureg.define('percent = 1/100')
ureg.define('permille = 1/1000')

val = 12 * ureg.percent
print(val.to('permille'))
# 12.0 permille
print(val.to('dimensionless'))
# 12.0 dimensionless

From what I found, this is because in the latter case, the unit is defined with is_base = True. It happens here because converter.keys() is empty.

I found out afterwards from #379 that I could use

ureg.define('percent = 0.01*count)
ureg.define('permille = 0.001*count)

but I figured I'd post this anway, at least for the import path update.

[lukelbd]

Thanks for the examples @lafrech.

I used your first method instead of #379 because then to_base_units() returns dimensionless units rather than count units, which conceptually makes more sense to me.

I also added "pre-processors" to the unit registry so that the symbols % and %% can be used as the canonical short forms for the percent and permille units (idea is taken from #429 but updated for the latest API). See the following code:

import pint
ureg = pint.UnitRegistry(preprocessors=[
    lambda s: s.replace('%%', ' permille '),
    lambda s: s.replace('%', ' percent '),
])
ureg.define(pint.unit.UnitDefinition(
    'permille', '%%', (), pint.converters.ScaleConverter(0.001),
))
ureg.define(pint.unit.UnitDefinition(
    'percent', '%', (), pint.converters.ScaleConverter(0.01),
))

This results in the following behavior:

>>> ureg('1%').to_base_units()
0.01 <Unit('dimensionless')>
>>> format(ureg('1%'), '~')
'1 %'
>>> format(ureg('1%%'), '~')
'1 %%'

[lukelbd]

Looks like the issue @lafrech mentioned has been fixed by version 0.16. The following now works:

import pint
ureg = pint.UnitRegistry(preprocessors=[
    lambda s: s.replace('%%', ' permille '),
    lambda s: s.replace('%', ' percent '),
])
ureg.define('permille = 0.001 = %%')  # or 'permille = 1 / 1000 = %%'
ureg.define('percent = 0.01 = %')

Results in:

>>> ureg('1 permille').to('')
0.001 <Unit('dimensionless')>

[mkaut]

With solution by @lukelbd,

x = ureg('1 percent').to('')

works and returns dimensionless 0.01, but

x.to('%')

does not - any way to get around this?

[ayding11]

I have the same issue as @mkaut. With the solution by @lukelbd, converting from "dimensionless" or "percent" to "%" does not work. I am using Pint 0.22. Any workaround? Thanks.

ureg.Quantity(5, 'dimensionless').to('%')

produces:
File "/usr/local/lib/python3.10/dist-packages/pint/facets/plain/quantity.py", line 515, in to other = to_units_container(other, self._REGISTRY) File "/usr/local/lib/python3.10/dist-packages/pint/util.py", line 1041, in to_units_container return registry._parse_units(unit_like) File "/usr/local/lib/python3.10/dist-packages/pint/facets/nonmultiplicative/registry.py", line 70, in _parse_units return super()._parse_units(input_string, as_delta, case_sensitive) File "/usr/local/lib/python3.10/dist-packages/pint/facets/plain/registry.py", line 1153, in _parse_units units = ParserHelper.from_string(input_string, self.non_int_type) File "/usr/local/lib/python3.10/dist-packages/pint/util.py", line 764, in from_string ret = build_eval_tree(gen).evaluate( File "/usr/local/lib/python3.10/dist-packages/pint/pint_eval.py", line 329, in build_eval_tree result, _ = _build_eval_tree(tokens, op_priority, 0, 0) File "/usr/local/lib/python3.10/dist-packages/pint/pint_eval.py", line 254, in _build_eval_tree right, index = _build_eval_tree( File "/usr/local/lib/python3.10/dist-packages/pint/pint_eval.py", line 278, in _build_eval_tree assert result is not None AssertionError