[AstroPy] why the warning about Converting Quantity?

Jonathan Slavin jslavin@cfa.harvard....
Fri Mar 22 11:37:51 CDT 2013


Ah! Now I see.  All I needed to do was:

AU = constants.au.to('km').value
xe = xem/AU

By just using the value attribute I get what I want.

Jon

On Fri, 2013-03-22 at 10:57 -0400, Erik Tollerud wrote:
> Hello Jonathan et al.,
> 
> I think some confusion here stems from the fact that ``AU`` is a bit
> of a special constant - in most cases, you're probably better off
> using the AU *unit*, rather than the constant.  That is, if you were
> doing something like
> 
> arrinau = arr * constants.au
> 
> That gives exactly the warning you mentioned, whereas
> 
> arrinau = arr * units.au
> 
> won't give out a warning, because it creates a `Quantity` object with
> the units `AU`, rather than a regular array without any units.
> 
> 
> Does that make sense, or am I still misunderstanding how you're using
> this? If the latter, can you give an actual example of the line that
> actually produces this warning? That'll make it easier to understand
> exactly what you're suggesting.
> 
> 
> 
> 
> On Thu, Mar 21, 2013 at 3:22 PM, Jonathan Slavin
> <jslavin@cfa.harvard.edu> wrote:
> > Hi Adrian,
> >
> > Yes, as I mentioned farther down in my post, the warnings are generated
> > when I use it in an expression -- with numpy arrays.  It'd be nice to
> > have a less cumbersome way of suppressing those warnings (yes it's just
> > one line, but not so easy to remember).  In fact, I would expect to use
> > the constants mostly in expressions with numpy arrays, variables, etc.
> > that don't have units, so my preference would be that by default those
> > warnings are turned off -- at least for the constants.  The constants
> > are useful for their values alone, though clearly the value of the units
> > module is to associate units with values.  Maybe a simple method like
> > value.asfloat() could be added that would allow conversion from a units
> > object to a numpy float.
> >
> > Jon
> >
> > On Thu, 2013-03-21 at 15:05 -0400, Adrian Price-Whelan wrote:
> >> Hey Jonathan --
> >>
> >> Are you sure that's the point in the code that's producing the
> >> warning? What operations are you doing with the AU object? When you
> >> use the .to() method, it still returns a Quantity object -- *not* the
> >> value of the original object in those units. If you then stick AU into
> >> any numpy function, it will work but will just extract the value from
> >> the object (hence the warning).
> >>
> >> See, for example:
> >>
> >> >>> import astropy.units as u
> >> >>> np.sqrt(15*u.km)
> >> WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> [astropy.units.quantity]
> >> 3.872983346207417
> >>
> >> To turn this off, you'll want to set:
> >> >>> u.quantity.WARN_IMPLICIT_NUMERIC_CONVERSION.set(False)
> >> >>> np.sqrt(15*u.km)
> >> 3.872983346207417
> >>
> >> Note that this is documented in the astropy.units.quantity section of
> >> the astropy documentation:
> >> http://docs.astropy.org/en/latest/units/quantity.html#converting-to-python-or-numpy-types
> >>
> >> Thanks,
> >> Adrian
> >>
> >> On Mar 21, 2013, at 2:20 PM, Jonathan Slavin wrote:
> >>
> >> > Hi,
> >> >
> >> > I have just used the constants module in astropy for the first time and
> >> > got 6 warnings from my code as a result:
> >> >
> >> > import astropy.constants as const
> >> > AU = const.au.to('km')
> >> >
> >> > leads to
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> >
> >> > It seems that I get the warning every place I use the value of AU.  Do I
> >> > have to turn off warnings  to prevent this?  Is there some other way to
> >> > deal with this?
> >> >
> >> > Jon
> >> > --
> >> > ______________________________________________________________
> >> > Jonathan D. Slavin              Harvard-Smithsonian CfA
> >> > jslavin@cfa.harvard.edu         60 Garden Street, MS 83
> >> > phone: (617) 496-7981           Cambridge, MA 02138-1516
> >> > cell: (781) 363-0035           USA
> >> > ______________________________________________________________
> >> >
> >> > _______________________________________________
> >> > AstroPy mailing list
> >> > AstroPy@scipy.org
> >> > http://mail.scipy.org/mailman/listinfo/astropy
> >>
> >> --
> >> Adrian Price-Whelan
> >> Department of Astronomy
> >> Columbia University
> >>
> >>
> >>
> >
> > --
> > ______________________________________________________________
> > Jonathan D. Slavin              Harvard-Smithsonian CfA
> > jslavin@cfa.harvard.edu         60 Garden Street, MS 83
> > phone: (617) 496-7981           Cambridge, MA 02138-1516
> >  cell: (781) 363-0035           USA
> > ______________________________________________________________
> >
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy@scipy.org
> > http://mail.scipy.org/mailman/listinfo/astropy
> 
> 
> 
> --
> Erik
> 
> On Thu, Mar 21, 2013 at 3:22 PM, Jonathan Slavin
> <jslavin@cfa.harvard.edu> wrote:
> > Hi Adrian,
> >
> > Yes, as I mentioned farther down in my post, the warnings are generated
> > when I use it in an expression -- with numpy arrays.  It'd be nice to
> > have a less cumbersome way of suppressing those warnings (yes it's just
> > one line, but not so easy to remember).  In fact, I would expect to use
> > the constants mostly in expressions with numpy arrays, variables, etc.
> > that don't have units, so my preference would be that by default those
> > warnings are turned off -- at least for the constants.  The constants
> > are useful for their values alone, though clearly the value of the units
> > module is to associate units with values.  Maybe a simple method like
> > value.asfloat() could be added that would allow conversion from a units
> > object to a numpy float.
> >
> > Jon
> >
> > On Thu, 2013-03-21 at 15:05 -0400, Adrian Price-Whelan wrote:
> >> Hey Jonathan --
> >>
> >> Are you sure that's the point in the code that's producing the
> >> warning? What operations are you doing with the AU object? When you
> >> use the .to() method, it still returns a Quantity object -- *not* the
> >> value of the original object in those units. If you then stick AU into
> >> any numpy function, it will work but will just extract the value from
> >> the object (hence the warning).
> >>
> >> See, for example:
> >>
> >> >>> import astropy.units as u
> >> >>> np.sqrt(15*u.km)
> >> WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> [astropy.units.quantity]
> >> 3.872983346207417
> >>
> >> To turn this off, you'll want to set:
> >> >>> u.quantity.WARN_IMPLICIT_NUMERIC_CONVERSION.set(False)
> >> >>> np.sqrt(15*u.km)
> >> 3.872983346207417
> >>
> >> Note that this is documented in the astropy.units.quantity section of
> >> the astropy documentation:
> >> http://docs.astropy.org/en/latest/units/quantity.html#converting-to-python-or-numpy-types
> >>
> >> Thanks,
> >> Adrian
> >>
> >> On Mar 21, 2013, at 2:20 PM, Jonathan Slavin wrote:
> >>
> >> > Hi,
> >> >
> >> > I have just used the constants module in astropy for the first time and
> >> > got 6 warnings from my code as a result:
> >> >
> >> > import astropy.constants as const
> >> > AU = const.au.to('km')
> >> >
> >> > leads to
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> > WARNING: Converting Quantity object in units 'km' to a Numpy array
> >> > [astropy.units.quantity]
> >> >
> >> > It seems that I get the warning every place I use the value of AU.  Do I
> >> > have to turn off warnings  to prevent this?  Is there some other way to
> >> > deal with this?
> >> >
> >> > Jon
> >> > --
> >> > ______________________________________________________________
> >> > Jonathan D. Slavin              Harvard-Smithsonian CfA
> >> > jslavin@cfa.harvard.edu         60 Garden Street, MS 83
> >> > phone: (617) 496-7981           Cambridge, MA 02138-1516
> >> > cell: (781) 363-0035           USA
> >> > ______________________________________________________________
> >> >
> >> > _______________________________________________
> >> > AstroPy mailing list
> >> > AstroPy@scipy.org
> >> > http://mail.scipy.org/mailman/listinfo/astropy
> >>
> >> --
> >> Adrian Price-Whelan
> >> Department of Astronomy
> >> Columbia University
> >>
> >>
> >>
> >
> > --
> > ______________________________________________________________
> > Jonathan D. Slavin              Harvard-Smithsonian CfA
> > jslavin@cfa.harvard.edu         60 Garden Street, MS 83
> > phone: (617) 496-7981           Cambridge, MA 02138-1516
> >  cell: (781) 363-0035           USA
> > ______________________________________________________________
> >
> > _______________________________________________
> > AstroPy mailing list
> > AstroPy@scipy.org
> > http://mail.scipy.org/mailman/listinfo/astropy
> 
> 
> 
> --
> Erik Tollerud

-- 
______________________________________________________________
Jonathan D. Slavin              Harvard-Smithsonian CfA
jslavin@cfa.harvard.edu         60 Garden Street, MS 83
phone: (617) 496-7981           Cambridge, MA 02138-1516
 cell: (781) 363-0035           USA
______________________________________________________________



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