Percent (%) sign

[crusaderky]

After defining:

import pint
ureg = pint.UnitRegistry()
ureg.define("% = 0.01 count = percent")
ureg.define("%% = 0.001 count = per_mille")
ureg.define("ppm = 1e-6 count = parts_per_million")

This works:

>>> ureg("0.1 percent").to("ppm")
1000.0 ppm

But % and %% don't:

>>> ureg("0.1%").to("ppm")
AttributeError: 'float' object has no attribute 'to'
>>> ureg("0.1%")
0.1

[jthielen]

I think this issue has been around for a while (see #429), and there are many different workarounds. For example, MetPy has worked around it outside of pint (see Unidata/MetPy#1057).

But, looking back, @hgrecco's comment about having the pre-processor be pluggable/extensible as a way to resolve this (#429 (comment)) connects this to #851 and #799. Based on all this, I think I'm going to try putting together a PR for an extensible pre-processor sometime soon.

[crusaderky]

Indeed duplicates #429