Skip to main content
`Ctrl`+`K`

# Fast In-Place Modification of Coordinates

For some applications the recommended method of Modifying Coordinate Objects In-place may not be fast enough due to the extensive validation performed in that process to ensure correctness. Likewise, you may find that creating another coordinate frame with different data using `realize_frame` does not meet your performance requirements.

For these high-performance situations, you can directly modify in-place the representation data in the frame object as shown in this example:

    >>> import astropy.units as u
    >>> from astropy.coordinates import SkyCoord
    >>> sc = SkyCoord([1,2],[3,4], unit='deg')
    >>> sc.data.lon[()] = [10, 20] * u.deg
    >>> sc.data.lat[1] = 40 * u.deg

    >>> sc.cache.clear()  # IMPORTANT TO DO THIS!

    >>> sc
    <SkyCoord (ICRS): (ra, dec) in deg
        [(10., 3.), (20., 40.)]>

Notice that the `.data` representation object uses different names for the components than in the coordinate object. If you wish to inspect the mapping between frame attributes (e.g., `.ra`) and representation attributes (e.g., `.lon`) you can look at the following dictionary:

    >>> sc.representation_component_names
    {'ra': 'lon', 'dec': 'lat', 'distance': 'distance'}

Warning

You must include the step to clear the cache as shown. Failing to do so will cause the object to be inconsistent and likely result in incorrect results. `SkyCoord` and `BaseCoordinateFrame` cache various kinds of information for performance reasons, so you need clear the cache so that the new representation values are used when required.

You should note 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. For example the following will appear to give a correct result but it does not actually modify the underlying representation data:

    >>> sc.ra[1] = 20 * u.deg  # THIS IS WRONG

This problem is related to the current implementation of performance-based caching and cannot be easily resolved.