[AstroPy] Bug in units decompose()?

[John Quinn]

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 in decompose(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 in decompose(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, p in zip(self.bases, self.powers):
   1960             if decompose and b not 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 in decompose(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 in decompose(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, p in zip(self.bases, self.powers):
   1960             if decompose and b not 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 in decompose(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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