Imposing the value of Frame Attributes

In certain circumstances you may want to override the value of one frame attribute with another. Examples of when this might be useful include when you are reading a series of frames from some source and need to change an attribute for a transform or calculation, or when you are using a package such as reproject where you don’t control the construction of the coordinate objects.

Take this example, where you have two coordinates where the obstime difference results in a very subtly different origin of the coordinate frame. For this example we are constructing these coordinates explicitly:

>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord, impose_frame_attributes

>>> coord1 = SkyCoord(10*u.deg, 20*u.deg, frame='hcrs', obstime='2026-01-01 00:00:00')
>>> coord2 = SkyCoord(20*u.deg, 30*u.deg, frame='hcrs', obstime='2026-01-01 00:00:00.001')

When you try and compute the separation an error is raised because of the origin shift:

>>> print(coord1.separation(coord2))
...
astropy.units.errors.UnitsError: The input HCRS coordinates do not have length units. This probably means you created coordinates with lat/lon but no distance.  Heliocentric<->ICRS transforms cannot function in this case because there is an origin shift.

Given the very small difference in observer location assuming that they are equal is likely valid to a high degree of precision:

>>> with impose_frame_attributes(obstime=coord1.obstime):
...     print(coord1.separation(coord2))
13d28m54.20360928s

The impose_frame_attributes decorator accepts keyword arguments which are any attribute on any frame class. So in our previous example of HCRS the obstime attribute. Note that all frame attributes with that name on all frames will be overridden with the value. For example if you impose equinox= then it will apply to both FK4 and FK5 and any other frames which define that attribute.