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In Place Modification of Coordinates
====================================
Coordinates are generally considered to be immutable. If you want to create
another coordinate frame with different data you should use
`~astropy.coordinates.BaseCoordinateFrame.realize_frame`, this is the safest way
to change the data in a frame as it creates a new frame with that data.
Creating a new frame can be relatively slow, however, particularly for scalar
coordinates. Hence some situations may require that data by changed in-place in
an already existing frame object. This modification can be done by
modifiying the values of the representation data as follows::
>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord
>>> c = SkyCoord([1,2],[3,4], unit='deg')
>>> c.data.lon[()] = [10, 20] * u.deg
>>> c
<SkyCoord (ICRS): (ra, dec) in deg
[( 10., 3.), ( 20., 4.)]>
This changes the longitude values of the frame. Unfortunately, doing just this
introduces problems: `~astropy.coordinates.SkyCoord` and
`~astropy.coordinates.BaseCoordinateFrame` cache various kinds of information to
speed up some repeated operations. So we need to tell the cache that it should
be cleared so that it can be re-calculated from the new data. This can be
achieved by doing::
>>> c.cache.clear()
It should be noted that the only way to modify the data in a frame is by using
the ``.data`` attribute directly and not the aliases for components on the frame
*i.e.* the following will not work:::
>>> c.ra[()] = 20 * u.deg
This is because a different representation object is used when acessing the
aliased component names. If you wish to inspect the mapping between frame
attributes i.e. ``.ra`` and representation attributes i.e. ``.lon`` you can look
at the following dictionary.::
>>> c.representation_component_names
OrderedDict([('ra', 'lon'), ('dec', 'lat'), ('distance', 'distance')])
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