more trouble with force_ndarray_like=True

[keewis]

Apparently, force_ndarray_like=True results in q._units containing numpy scalar values, which causes unit conversions to fail:

In [1]: import pint
   ...: 
   ...: ureg = pint.UnitRegistry(force_ndarray_like=True)
   ...: q = ureg.Quantity(1, "1 / hours") ** 2
   ...: display({k: type(v) for k, v in q._units.items()})
   ...: q.to("1 / s ** 2")
{'hour': numpy.int64}
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-fd499ff53bb2> in <module>
      3 ureg = pint.UnitRegistry(force_ndarray_like=True)
      4 q = 1 / ureg.Quantity(1, "hours") ** 2
----> 5 q.to("1 / s ** 2")

.../pint/quantity.py in to(self, other, *contexts, **ctx_kwargs)
    655         other = to_units_container(other, self._REGISTRY)
    656 
--> 657         magnitude = self._convert_magnitude_not_inplace(other, *contexts, **ctx_kwargs)
    658 
    659         return self.__class__(magnitude, other)

.../pint/quantity.py in _convert_magnitude_not_inplace(self, other, *contexts, **ctx_kwargs)
    604                 return self._REGISTRY.convert(self._magnitude, self._units, other)
    605 
--> 606         return self._REGISTRY.convert(self._magnitude, self._units, other)
    607 
    608     def _convert_magnitude(self, other, *contexts, **ctx_kwargs):

.../pint/registry.py in convert(self, value, src, dst, inplace)
    945             return value
    946 
--> 947         return self._convert(value, src, dst, inplace)
    948 
    949     def _convert(self, value, src, dst, inplace=False, check_dimensionality=True):

.../pint/registry.py in _convert(self, value, src, dst, inplace)
   1828                 value, src = src._magnitude, src._units
   1829 
-> 1830         return super()._convert(value, src, dst, inplace)
   1831 
   1832     def _get_compatible_units(self, input_units, group_or_system):

.../pint/registry.py in _convert(self, value, src, dst, inplace)
   1431 
   1432         if not (src_offset_unit or dst_offset_unit):
-> 1433             return super()._convert(value, src, dst, inplace)
   1434 
   1435         src_dim = self._get_dimensionality(src)

.../pint/registry.py in _convert(self, value, src, dst, inplace, check_dimensionality)
    982         # Here src and dst have only multiplicative units left. Thus we can
    983         # convert with a factor.
--> 984         factor, _ = self._get_root_units(src / dst)
    985 
    986         # factor is type float and if our magnitude is type Decimal then

.../pint/registry.py in _get_root_units(self, input_units, check_nonmult)
    822 
    823         accumulators = [1, defaultdict(int)]
--> 824         self._get_root_units_recurse(input_units, 1, accumulators)
    825 
    826         factor = accumulators[0]

.../pint/registry.py in _get_root_units_recurse(self, ref, exp, accumulators)
    871                 accumulators[1][key] += exp2
    872             else:
--> 873                 accumulators[0] *= reg._converter.scale ** exp2
    874                 if reg.reference is not None:
    875                     self._get_root_units_recurse(reg.reference, exp2, accumulators)

ValueError: Integers to negative integer powers are not allowed.

In [2]: q._units = pint.util.to_units_container({k: getattr(v, "item", lambda: v)() for k, v in q._units.items()})
   ...: print(q.to("1 / s ** 2"))
7.71604938271605e-08 / second ** 2

I'm pretty sure this is unrelated to #1301 (although they share a common cause: numpy.int64(1) ** -2 raises) because this also fails if I go back to the release commit of 0.17.

[keewis]

this can be fixed by:

diff --git a/pint/quantity.py b/pint/quantity.py
index cbe181f..8704971 100644
--- a/pint/quantity.py
+++ b/pint/quantity.py
@@ -1486,7 +1486,7 @@ class Quantity(PrettyIPython, SharedRegistryObject):
                     raise DimensionalityError(other._units, "dimensionless")
                 else:
                     exponent = _to_magnitude(
-                        other, self.force_ndarray, self.force_ndarray_like
+                        other, force_ndarray=False, force_ndarray_like=False
                     )
                     units = new_self._units ** exponent

not sure if I'm missing something? The test suite at least doesn't fail.

[jules-ch]

Good catch.
Seems like the right thing to do.

Following my comment #60 (comment), IMO I think we need to constraint exponent type in UnitContainers

[keewis]

I would support that, but we have to make sure not to break the numpy support (does ureg.Quantity([0, 1], "m") ** Fraction("1.5") work?)