Astropy 1.1.1 version breaks Tesla conversion to cgs again

[fredgent]

With astropy 1.1.1 the conversion between Tesla and Gauss is once again broken. Tesla can be directly converted to Gauss and vice versa, but the
fv.attrs['field_to_cgs'] = field.unit.to_system(u.cgs)[0].scale
in the gdf_writer has no valid result

I have loaded astropy version 1.0.5 on iceberg, but we should raise this issue with the astropy developers. Clearly the bug was fixed last year and the latest upgrade has reintroduced it

[Cadair]

a simple example:

field = u.Quantity([1,2,3])*u.T

field.unit.to_system(u.cgs)
---------------------------------------------------------------------------
UnitConversionError                       Traceback (most recent call last)
<ipython-input-6-6c708e450c60> in <module>()
----> 1 field.unit.to_system(u.cgs)

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in to_system(self, system)
   1282                 return sum / float(len(compose_bases))
   1283 
-> 1284         x = self.decompose(bases=bases)
   1285         composed = x.compose(units=system)
   1286         composed = sorted(composed, key=score, reverse=True)

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   1925 
   1926     def decompose(self, bases=set()):
-> 1927         return self._represents.decompose(bases=bases)
   1928 
   1929     def is_unity(self):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   2102 
   2103         x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
-> 2104                           decompose_bases=bases)
   2105         if len(bases) == 0:
   2106             self._decomposed_cache = x

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in __init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
   1985         self._powers = powers
   1986         self._decomposed_cache = None
-> 1987         self._expand_and_gather(decompose=decompose, bases=decompose_bases)
   1988         self._hash = None
   1989 

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in _expand_and_gather(self, decompose, bases)
   2058         for b, p in zip(self.bases, self.powers):
   2059             if decompose and b not in bases:
-> 2060                 b = b.decompose(bases=bases)
   2061 
   2062             if isinstance(b, CompositeUnit):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   1925 
   1926     def decompose(self, bases=set()):
-> 1927         return self._represents.decompose(bases=bases)
   1928 
   1929     def is_unity(self):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   2102 
   2103         x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
-> 2104                           decompose_bases=bases)
   2105         if len(bases) == 0:
   2106             self._decomposed_cache = x

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in __init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
   1985         self._powers = powers
   1986         self._decomposed_cache = None
-> 1987         self._expand_and_gather(decompose=decompose, bases=decompose_bases)
   1988         self._hash = None
   1989 

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in _expand_and_gather(self, decompose, bases)
   2058         for b, p in zip(self.bases, self.powers):
   2059             if decompose and b not in bases:
-> 2060                 b = b.decompose(bases=bases)
   2061 
   2062             if isinstance(b, CompositeUnit):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   1925 
   1926     def decompose(self, bases=set()):
-> 1927         return self._represents.decompose(bases=bases)
   1928 
   1929     def is_unity(self):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   2102 
   2103         x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
-> 2104                           decompose_bases=bases)
   2105         if len(bases) == 0:
   2106             self._decomposed_cache = x

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in __init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
   1985         self._powers = powers
   1986         self._decomposed_cache = None
-> 1987         self._expand_and_gather(decompose=decompose, bases=decompose_bases)
   1988         self._hash = None
   1989 

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in _expand_and_gather(self, decompose, bases)
   2058         for b, p in zip(self.bases, self.powers):
   2059             if decompose and b not in bases:
-> 2060                 b = b.decompose(bases=bases)
   2061 
   2062             if isinstance(b, CompositeUnit):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   1925 
   1926     def decompose(self, bases=set()):
-> 1927         return self._represents.decompose(bases=bases)
   1928 
   1929     def is_unity(self):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   2102 
   2103         x = CompositeUnit(self.scale, self.bases, self.powers, decompose=True,
-> 2104                           decompose_bases=bases)
   2105         if len(bases) == 0:
   2106             self._decomposed_cache = x

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in __init__(self, scale, bases, powers, decompose, decompose_bases, _error_check)
   1985         self._powers = powers
   1986         self._decomposed_cache = None
-> 1987         self._expand_and_gather(decompose=decompose, bases=decompose_bases)
   1988         self._hash = None
   1989 

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in _expand_and_gather(self, decompose, bases)
   2058         for b, p in zip(self.bases, self.powers):
   2059             if decompose and b not in bases:
-> 2060                 b = b.decompose(bases=bases)
   2061 
   2062             if isinstance(b, CompositeUnit):

/opt/miniconda/envs/sunpy-dev/lib/python2.7/site-packages/astropy/units/core.pyc in decompose(self, bases)
   1660             raise UnitConversionError(
   1661                 "Unit {0} can not be decomposed into the requested "
-> 1662                 "bases".format(self))
   1663 
   1664         return self

UnitConversionError: Unit A can not be decomposed into the requested bases

[Cadair]

this is the fault of astropy/astropy#4355 and the fact it was backported to 1.1.1.

[Cadair]

I guess the fix is to convert directly to Gauss

[SolarDrew]

I did try converting to Gauss and it doesn't actually seem to get around the problem, unless I missed something.

[Cadair]

hot damn.