Create ndarray (recarray?) subclass that can hold multiple Quantities of mixed units #3777
Description
Over in this comment, @mhvk suggested that if we want an array-like container for Quantities of different units, if nothing else we could just use an ndarray of dtype=object to hold multiple Quantity objects. And in principle I'm fine with that as a solution.
However, I suggested in a followup that a subclass specially for this purpose might still be useful, if nothing else, for improved representation. For example, instead of an array of Quantities looking like:
[<Quantity 1.0 m>, <Quantity 2.0 km>, <Quantity 3.0 Jy>](which looks even worse when you go > 1 dimension), it might be nice to have something that reprs/prints like
[1.0 m, 2.0 m, 3.0 Jy]etc. This would look especially nice if the repr is justified so that the columns line up :)
However, it turns out that a subclass for this purpose might be necessary even aside from aesthetic improvements, since it seems currently (at least with Numpy 1.9.1 which I've tested this on) it's not even possible to repr an array of Quantities:
In [23]: a = np.array([1 * u.km, 2 * u.J], dtype=object)
In [24]: print(a)
---------------------------------------------------------------------------
UnitsError Traceback (most recent call last)
<ipython-input-24-c5a4f3535135> in <module>()
----> 1 print(a)
C:\Python27\lib\site-packages\numpy\core\numeric.pyc in array_str(a, max_line_width, precision, suppress_small)
1713
1714 """
-> 1715 return array2string(a, max_line_width, precision, suppress_small, '
', "", str)
1716
1717 def set_string_function(f, repr=True):
C:\Python27\lib\site-packages\numpy\core\arrayprint.pyc in array2string(a, max_line_width, precision, suppress_small, separator, prefix, style, formatter)
452 else:
453 lst = _array2string(a, max_line_width, precision, suppress_small,
--> 454 separator, prefix, formatter=formatter)
455 return lst
456
C:\Python27\lib\site-packages\numpy\core\arrayprint.pyc in _array2string(a, max_line_width, precision, suppress_small, separator, prefix, formatter)
254
255 formatdict = {'bool' : _boolFormatter,
--> 256 'int' : IntegerFormat(data),
257 'float' : FloatFormat(data, precision, suppress_small),
258 'longfloat' : LongFloatFormat(precision),
C:\Python27\lib\site-packages\numpy\core\arrayprint.pyc in __init__(self, data)
639 def __init__(self, data):
640 try:
--> 641 max_str_len = max(len(str(maximum.reduce(data))),
642 len(str(minimum.reduce(data))))
643 self.format = '%' + str(max_str_len) + 'd'
C:\MinGW\msys\1.0\home\embray\src\astropy\astropy\units\quantity.pyc in __array_prepare__(self, obj, context)
293 # the unit the output from the ufunc will have.
294 if function in UFUNC_HELPERS:
--> 295 converters, result_unit = UFUNC_HELPERS[function](function,
*units)
296 else:
297 raise TypeError("Unknown ufunc {0}. Please raise issue on "
C:\MinGW\msys\1.0\home\embray\src\astropy\astropy\units\quantity_helper.pyc in helper_twoarg_comparison(f, unit1, unit2)
305
306 def helper_twoarg_comparison(f, unit1, unit2):
--> 307 converters, _ = get_converters_and_unit(f, unit1, unit2)
308 return converters, None
309
C:\MinGW\msys\1.0\home\embray\src\astropy\astropy\units\quantity_helper.pyc in get_converters_and_unit(f, *units)
283 "Can only apply '{0}' function to quantities "
284 "with compatible dimensions"
--> 285 .format(f.__name__))
286
287 return converters, units[fixed]
UnitsError: Can only apply 'less' function to quantities with compatible dimensionsIt seems that the Numpy array print code at some point tries ordering the values in the array, leading to this crash. This indicates maybe two things:
-
We could work around this specific error, I think, by making
UnitsErroraValueErrorsubclass. -
Even if we fixed that specific issue it demonstrates that an array of Quantities can't be displays as well as one might like, and it might be worth fixing that.
A further question is why would we even need an array of Quantities of different units. I think if nothing else Quantity support in modeling may end up wanting something like this.