units: 's / m' and 's / m' are not convertible

@astrofrog

To paraphrase from PlasmaPy/PlasmaPy#587, exceptions like astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible (and a few other cases with first, second and third powers of second over meter, curiously) started popping up in our travis tests as seen at https://travis-ci.org/PlasmaPy/PlasmaPy/jobs/466396211 .

For a brief overview, running python setup.py test -d and getting into plasmapy.physics.distribution.Maxwellian_1D:

(Pdb) distFunc
<Quantity 5.91632969e-07 s / m>
(Pdb) p u.s / u.m
Unit("s / m")
(Pdb) p distFunc
<Quantity 5.91632969e-07 s / m>
(Pdb) p distFunc.unit
Unit("s / m")
(Pdb) p distFunc.to(u.s / u.m)
*** astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible

I've managed to figure out that this is localized to astropy 3.1. As suggested by @astrofrog on slack, I ran git bisect with pytest --doctest-modules $PLASMAPY/plasmapy/physics/distribution.py and I was able to identify the first bad commit as

3a478ca29e86144d6c5a0305dde86169a647ff63 is the first bad commit
commit 3a478ca29e86144d6c5a0305dde86169a647ff63
Author: Marten van Kerkwijk <[email protected]>
Date:   Thu Jul 12 21:01:28 2018 -0700

    Further short-cuts for single-unit CompositeUnit initialization.

:040000 040000 1ba7cb9440215ab641197f91ef1623ad9bb39378 5c02db4ede9146ec12f84f57a0382f4900777544 M      astropy

I'm currently trying to find out the exact cause of this bug in that commit, but I figured it's best to submit this issue here before I accidentally close this tab or something.

@mhvk

Thanks for the details. That commit was part of #7649 . cc @mhvk

Here's a more detailed traceback done from 3a478ca:

plasmapy/physics/tests/test_distribution.py:21 (test_astropy)
def test_astropy():
        v=1*u.m/u.s
>       Maxwellian_1D(v=v, T=30000 * u.K, particle='e', v_drift=0 * u.m / u.s)

plasmapy/physics/tests/test_distribution.py:24: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
plasmapy/physics/distribution.py:142: in Maxwellian_1D
    return distFunc.to(u.s / u.m)
../../astropy/astropy/units/quantity.py:669: in to
    return self._new_view(self._to_value(unit, equivalencies), unit)
../../astropy/astropy/units/quantity.py:641: in _to_value
    equivalencies=equivalencies)
../../astropy/astropy/units/core.py:984: in to
    return self._get_converter(other, equivalencies=equivalencies)(value)
../../astropy/astropy/units/core.py:915: in _get_converter
    raise exc
../../astropy/astropy/units/core.py:901: in _get_converter
    self, other, self._normalize_equivalencies(equivalencies))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = Unit("s / m"), unit = Unit("s / m"), other = Unit("s / m")
equivalencies = []

    def _apply_equivalencies(self, unit, other, equivalencies):
        """
        Internal function (used from `_get_converter`) to apply
        equivalence pairs.
        """
        def make_converter(scale1, func, scale2):
            def convert(v):
                return func(_condition_arg(v) / scale1) * scale2
            return convert
    
        for funit, tunit, a, b in equivalencies:
            if tunit is None:
                try:
                    ratio_in_funit = (other.decompose() /
                                      unit.decompose()).decompose([funit])
                    return make_converter(ratio_in_funit.scale, a, 1.)
                except UnitsError:
                    pass
            else:
                try:
                    scale1 = funit._to(unit)
                    scale2 = tunit._to(other)
                    return make_converter(scale1, a, scale2)
                except UnitsError:
                    pass
                try:
                    scale1 = tunit._to(unit)
                    scale2 = funit._to(other)
                    return make_converter(scale1, b, scale2)
                except UnitsError:
                    pass
    
        def get_err_str(unit):
            unit_str = unit.to_string('unscaled')
            physical_type = unit.physical_type
            if physical_type != 'unknown':
                unit_str = "'{0}' ({1})".format(
                    unit_str, physical_type)
            else:
                unit_str = "'{0}'".format(unit_str)
            return unit_str
    
        unit_str = get_err_str(unit)
        other_str = get_err_str(other)
    
        raise UnitConversionError(
            "{0} and {1} are not convertible".format(
>               unit_str, other_str))
E       astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible

../../astropy/astropy/units/core.py:885: UnitConversionError

I think I've got something. At the end of the problematic Maxwellian_1D function, we have a return distFunc.to(u.s / u.m). In what follows, unit is the unit of distFunc and other is u.s / u.m:

(plasmapy-tests) 18:07:23 dominik: ~/Code/PlasmaPy/PlasmaPy $ pytest --doctest-modules --pdb plasmapy/physics/distribution.py  
=========================================================================== test session starts ============================================================================
platform linux -- Python 3.7.0, pytest-4.0.1, py-1.7.0, pluggy-0.8.0
rootdir: /home/dominik/Code/PlasmaPy/PlasmaPy, inifile: setup.cfg
plugins: remotedata-0.3.1, openfiles-0.3.1, doctestplus-0.2.0, arraydiff-0.2
collected 8 items                                                                                                                                                          

plasmapy/physics/distribution.py F
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> traceback >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
098         \equiv \frac{1}{\sqrt{\pi v_{Th}^2}} e^{-(v - v_{drift})^2 / v_{Th}^2}
099 
100     where :math:`v_{Th} = \sqrt{2 k_B T / m}` is the thermal speed
101 
102     Examples
103     --------
104     >>> from plasmapy.physics import Maxwellian_1D
105     >>> from astropy import units as u
106     >>> v=1*u.m/u.s
107     >>> Maxwellian_1D(v=v, T=30000 * u.K, particle='e', v_drift=0 * u.m / u.s)
UNEXPECTED EXCEPTION: UnitConversionError("'s / m' and 's / m' are not convertible")
Traceback (most recent call last):

  File "/home/dominik/.miniconda3/envs/plasmapy-tests/lib/python3.7/doctest.py", line 1329, in __run
    compileflags, 1), test.globs)

  File "<doctest plasmapy.physics.distribution.Maxwellian_1D[3]>", line 1, in <module>

  File "/home/dominik/Code/PlasmaPy/PlasmaPy/plasmapy/physics/distribution.py", line 142, in Maxwellian_1D
    return distFunc.to(u.s / u.m)

  File "/home/dominik/Code/astropy/astropy/units/quantity.py", line 669, in to
    return self._new_view(self._to_value(unit, equivalencies), unit)

  File "/home/dominik/Code/astropy/astropy/units/quantity.py", line 641, in _to_value
    equivalencies=equivalencies)

  File "/home/dominik/Code/astropy/astropy/units/core.py", line 984, in to
    return self._get_converter(other, equivalencies=equivalencies)(value)

  File "/home/dominik/Code/astropy/astropy/units/core.py", line 915, in _get_converter
    raise exc

  File "/home/dominik/Code/astropy/astropy/units/core.py", line 901, in _get_converter
    self, other, self._normalize_equivalencies(equivalencies))

  File "/home/dominik/Code/astropy/astropy/units/core.py", line 885, in _apply_equivalencies
    unit_str, other_str))

astropy.units.core.UnitConversionError: 's / m' and 's / m' are not convertible

/home/dominik/Code/PlasmaPy/PlasmaPy/plasmapy/physics/distribution.py:107: UnexpectedException
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> entering PDB >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
> /home/dominik/Code/astropy/astropy/units/core.py(885)_apply_equivalencies()
-> unit_str, other_str))
(Pdb) p vars(unit)
{'_bases': [Unit("m"), Unit("s")], '_powers': [-1.0, 1.0], '_scale': 1.0, '_decomposed_cache': Unit("s / m")}
(Pdb) p vars(other)
{'_scale': 1.0, '_bases': [Unit("s"), Unit("m")], '_powers': [1, -1], '_decomposed_cache': Unit("s / m")}

So I think this has something to do with that the fact that _powers are floats in one case and ints in another. It may also have to do with the fact that _bases don't have the same ordering and thus you can't simply (as I assume this does somewhere... haven't been able to track it down) cast powers to a common numeric type and check if they agree. They have to be sorted with the same ordering that sorts _bases first.

Damn, and here I just moved the units module indicator to stable! I'll try to trace down further (the hints certainly are helpful!)

OK, here is an astropy-only version (proving it is purely an astropy bug):

import astropy.units as u
v2 = 1*u.m**2/u.s**2
(v2 ** (-1/2)).to(u.s/u.m)
# UnitConversionError: 's / m' and 's / m' are not convertible

The real problem is that the order of the powers is flipped, which means the bases are not sorted.

Fix bug with raising to a negative power.

Fix in #8263. This bug does show the risks of trying to special-case code for speed...

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units: 's / m' and 's / m' are not convertible #8260

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units: 's / m' and 's / m' are not convertible #8260

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pllim commented on Dec 11, 2018

StanczakDominik commented on Dec 11, 2018

StanczakDominik commented on Dec 11, 2018

mhvk commented on Dec 11, 2018

mhvk commented on Dec 11, 2018

mhvk commented on Dec 11, 2018

mhvk commented on Dec 11, 2018

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