[AstroPy] Bug in units decompose()?
John Quinn
john.quinn at ucd.ie
Tue Jul 1 16:14:16 EDT 2014
Thanks for the responses.
I think I can use to() to convert to other units and that will do what I want.
I was confused after reading the documentation at:
http://docs.astropy.org/en/stable/units/decomposing_and_composing.html#reducing-a-unit-to-its-irreducible-parts
and found that:
> u.J.compose()
[Unit("J"), Unit("1e+07 erg"), Unit("4.58743e+17 Ry"), Unit("6.24151e+18 eV")]
> u.Ry.decompose(bases=[u.m, u.N])
2.17987e-18 m N
> u.eV.decompose(bases=[u.m, u.N])
1.60218e-19 m N
but:
> u.erg.decompose(bases=[u.m, u.N])
gives an error.
Similarly, converting to J sometimes works and sometimes doesn't, and I was very surprised that decomposing
something in J to J gave an error.
Regards,
John
On 1 Jul 2014, at 18:43, Erik Bray <embray at stsci.edu> wrote:
> 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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>>
>
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Dr. John Quinn
School of Physics
Science Center North
University College Dublin
Belfield, Dublin 4
Ireland
T: +353-1-7162278
F: +353-1-2837275
john.quinn at ucd.ie
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