[AstroPy] Bug in units decompose()?

[Erik Bray]

If I understand correctly this is because u.J is already defined in terms of 
kilograms, meters, and seconds (actually Newtons and meters).  You could 
redefine J as a new unit, and then redefine those other units relative to the new J.

But I'm not entirely clear exactly what the intent is here.  I do agree the 
error is a little confusing. But not as much if you know that Joule is already a 
compound unit, and it's trying to recursively decompose the units that comprise 
it in terms of Joules, hence the confusion.

Erik

On 07/01/2014 11:18 AM, John Quinn wrote:
> Hello,
>
> I am currently evaluating iPython Notebook as a replacement for MATHCAD in our
> teaching laboratories
> and am particularly interested in using the units and constants packages of
> astropy.
>
> I have encountered an apparent bug using the units decompose function and have
> reduced it to the
> following example:
>
>  > from astropy import units as u
>  > u.J.decompose(bases=[u.J])
>
> which produces the errors given below.
>
> Similarly, I cannot decompose a unit consisting of 1 u.N * u.m into u.J, but
> strangely u.Ry.decompose(bases=[u.J]) works fine.
>
> Am I missing something obvious or is this a bug?
>
> I am using astropy 0.3.2 with python 3.4.1 on Mac OS X  (anaconda). I see the
> same on the Linux and Windows
> versions and with Python 2.7.7.
>
> Thanks,
>
> John
>
> ---------------------------------------------------------------------------
> UnitsError                                 Traceback (most recent call last)
> <ipython-input-54-3aa084c7cc08>  in<module>()
> ----> 1  u.J.decompose(bases=[u.J])
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  indecompose(self, bases)
>     1837
>     1838      def  decompose(self,  bases=set()):
> -> 1839          return  self._represents.decompose(bases=bases)
>     1840      decompose.__doc__=  UnitBase.decompose.__doc__
>     1841
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  indecompose(self, bases)
>     1999
>     2000          x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
> -> 2001                            decompose_bases=bases)
>     2002          if  len(bases)  ==  0:
>     2003              self._decomposed_cache=  x
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  in__init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
>     1897          self._powers=  powers
>     1898          self._decomposed_cache=  None
> -> 1899          self._expand_and_gather(decompose=decompose,  bases=decompose_bases)
>     1900
>     1901      def  __repr__(self):
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  in_expand_and_gather(self, decompose, bases)
>     1959          for  b,  pin  zip(self.bases,  self.powers):
>     1960              if  decomposeand  bnot  in  bases:
> -> 1961                  b  =  b.decompose(bases=bases)
>     1962
>     1963              if  isinstance(b,  CompositeUnit):
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  indecompose(self, bases)
>     1837
>     1838      def  decompose(self,  bases=set()):
> -> 1839          return  self._represents.decompose(bases=bases)
>     1840      decompose.__doc__=  UnitBase.decompose.__doc__
>     1841
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  indecompose(self, bases)
>     1999
>     2000          x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
> -> 2001                            decompose_bases=bases)
>     2002          if  len(bases)  ==  0:
>     2003              self._decomposed_cache=  x
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  in__init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
>     1897          self._powers=  powers
>     1898          self._decomposed_cache=  None
> -> 1899          self._expand_and_gather(decompose=decompose,  bases=decompose_bases)
>     1900
>     1901      def  __repr__(self):
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  in_expand_and_gather(self, decompose, bases)
>     1959          for  b,  pin  zip(self.bases,  self.powers):
>     1960              if  decomposeand  bnot  in  bases:
> -> 1961                  b  =  b.decompose(bases=bases)
>     1962
>     1963              if  isinstance(b,  CompositeUnit):
>
> /Users/quinn/anaconda/envs/py34/lib/python3.4/site-packages/astropy/units/core.py  indecompose(self, bases)
>     1573              raise UnitsError(
>     1574                  "Unit {0} can not be decomposed into the requested"
> -> 1575                  "bases".format(self))
>     1576
>     1577          return  self
>
> UnitsError: Unit kg can not be decomposed into the requested bases
>
>
>
>
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