Applying numpy.cbrt (cubic root) loses quantity

[omtee]

pint.__version__
'0.9'
a = ur.Quantity(100, "m ** 3")
print(a)
100 meter ** 3
print(np.sqrt(a))
10.0 meter ** 1.5
print(np.cbrt(a))
4.641588833612779

[hgrecco]

Yes. This happens because sqrt is handled by pint. I hope that we the recent addition to numpy all these quirks will be removed.

[sxie22]

When trying to find a workaround, I found that adding another dictionary entry "cbrt" : 1/3 here in quantity.py seems to work.

pint/pint/quantity.py

Line 1321 in d804d93

'sqrt': .5, 'square': 2, 'reciprocal': -1}

from pint import UnitRegistry
ur = UnitRegistry()
a = ur.Quantity(100, "m ** 3")
print(a)
100 meter ** 3
print(np.sqrt(a))
10.0 meter ** 1.5
print(np.cbrt(a))
4.641588833612778 meter

Could you look into this as a hotfix?

[jthielen]

This was not added in #905 (even though it likely would have been easy to do so in retrospect). However, this should be an easy addition by defining a new unit operation in

pint/pint/numpy_func.py

Lines 94 to 152 in 43fbae2

	def get_op_output_unit(unit_op, first_input_units, all_args=[], size=None):
	"""Determine resulting unit from given operation.
	Options for `unit_op`:
	- "sum": `first_input_units`, unless non-multiplicative, which raises
	OffsetUnitCalculusError
	- "mul": product of all units in `all_args`
	- "delta": `first_input_units`, unless non-multiplicative, which uses delta version
	- "delta,div": like "delta", but divided by all units in `all_args` except the first
	- "div": unit of first argument in `all_args` (or dimensionless if not a Quantity) divided
	by all following units
	- "variance": square of `first_input_units`, unless non-multiplicative, which raises
	OffsetUnitCalculusError
	- "square": square of `first_input_units`
	- "sqrt": square root of `first_input_units`
	- "reciprocal": reciprocal of `first_input_units`
	- "size": `first_input_units` raised to the power of `size`
	"""
	if unit_op == "sum":
	result_unit = (1 * first_input_units + 1 * first_input_units).units
	elif unit_op == "mul":
	product = first_input_units._REGISTRY.parse_units('')
	for x in all_args:
	if hasattr(x, 'units'):
	product *= x.units
	result_unit = product
	elif unit_op == "delta":
	result_unit = (1 * first_input_units - 1 * first_input_units).units
	elif unit_op == "delta,div":
	product = (1 * first_input_units - 1 * first_input_units).units
	for x in all_args[1:]:
	if hasattr(x, 'units'):
	product /= x.units
	result_unit = product
	elif unit_op == "div":
	# Start with first arg in numerator, all others in denominator
	product = getattr(all_args[0], 'units', first_input_units._REGISTRY.parse_units(''))
	for x in all_args[1:]:
	if hasattr(x, 'units'):
	product /= x.units
	result_unit = product
	elif unit_op == "variance":
	result_unit = ((1 * first_input_units + 1 * first_input_units)**2).units
	elif unit_op == "square":
	result_unit = first_input_units**2
	elif unit_op == "sqrt":
	result_unit = first_input_units**0.5
	elif unit_op == "reciprocal":
	result_unit = first_input_units**-1
	elif unit_op == "size":
	if size is None:
	raise ValueError('size argument must be given when unit_op=="size"')
	result_unit = first_input_units**size
	else:
	raise ValueError('Output unit method {} not understood'.format(unit_op))
	return result_unit

and adding the appropriate entry in:

pint/pint/numpy_func.py

Lines 266 to 269 in 43fbae2

	op_units_output_ufuncs = {'var': 'square', 'prod': 'size', 'multiply': 'mul',
	'true_divide': 'div', 'divide': 'div', 'floor_divide': 'div',
	'sqrt': 'sqrt', 'square': 'square', 'reciprocal': 'reciprocal',
	'std': 'sum', 'sum': 'sum', 'cumsum': 'sum', 'matmul': 'mul'}