How to define and use percentage in Pint

Question

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 1

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

Answer 2

I had the same problem, and came up with a simple solution which seems to work well.

import pint

ureg = pint.UnitRegistry()
ureg.define('fraction = [] = frac')
ureg.define('percent = 1e-2 frac = pct')
ureg.define('ppm = 1e-6 fraction')

print(ureg('100 pct').to('dimensionless'))
print(ureg('0.5 dimensionless').to('pct'))
print(ureg('pct').to('ppm'))
print(ureg('1e4 ppm').to('pct'))

Output:

1.0 dimensionless
50.0 percent
10000.0 ppm
0.9999999999999999 percent

(Note slight rounding error on last line.)

I got the syntax from the default units dictionary, where base units are defined like

# reference
gram = [mass] = g  # dimensional
radian = [] = rad  # dimensionless