CoryKramer CoryKramer - 3 months ago 10
Python Question

How to define and use percentage in Pint

I'm currently using Pint to handle units and unit conversions. This seems to work well for the units that are already defined in Pint, for example

>>> import pint
>>> ureg = pint.UnitRegistry()
>>> Q = ureg.Quantity
>>> a = Q(5, 'm/s')
>>> a
<Quantity(5, 'meter / second')>
>>> a.to('ft/s')
<Quantity(16.404199475065617, 'foot / second')>


I tried to define my own units, which represent percentage. As far as unit conversions go, a percentage is simply 100 times a dimensionless fraction, which is how I defined it.

>>> ureg.define('percent = dimensionless * 100 = pct')
>>> a = Q(5, 'pct')
>>> a
<Quantity(5, 'percent')>


However I cannot seem to convert back and forth between fraction (
'dimensionless'
) and
'pct'
.

>>> a.to('dimensionless')
Traceback (most recent call last):
File "<pyshell#31>", line 1, in <module>
a.to('dimensionless')
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\quantity.py", line 263, in to
magnitude = self._convert_magnitude_not_inplace(other, *contexts, **ctx_kwargs)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\quantity.py", line 231, in _convert_magnitude_not_inplace
return self._REGISTRY.convert(self._magnitude, self._units, other)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\unit.py", line 1026, in convert
return self._convert(value, src, dst, inplace)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\unit.py", line 1042, in _convert
src_dim = self._get_dimensionality(src)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\unit.py", line 813, in _get_dimensionality
self._get_dimensionality_recurse(input_units, 1.0, accumulator)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\unit.py", line 837, in _get_dimensionality_recurse
self._get_dimensionality_recurse(reg.reference, exp2, accumulator)
File "C:\Python35\python-3.5.1.amd64\lib\site-packages\pint\unit.py", line 835, in _get_dimensionality_recurse
reg = self._units[self.get_name(key)]
KeyError: ''


What I'd essentially like to do is be able to convert between e.g. "0.73" and "73%". How can I define and use such a unit?

Answer

It seems that GitHub issue hgrecco/pint#185 covers the case you're describing.

Using the work-around discussed in that issue works for me using Pint-0.7.2:

from pint.unit import ScaleConverter
from pint.unit import UnitDefinition
import pint

ureg = pint.UnitRegistry()
Q = ureg.Quantity

ureg.define(UnitDefinition('percent', 'pct', (), ScaleConverter(1 / 100.0)))
a = Q(5, 'pct')

print a
print a.to('dimensionless')

Output:

5 percent
0.05 dimensionless