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TASK OBJECTIVE

astropy/astropy

How do you make wcs_pix2world([], [], 0) return empty arrays instead of raising InconsistentAxisTypesError?

InconsistentAxisTypesErrorPython traceback repairs

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01

TASK STATEMENT

verbatim agent-facing issue statement

Issue when passing empty lists/arrays to WCS transformations
The following should not fail but instead should return empty lists/arrays:

```
In [1]: from astropy.wcs import WCS

In [2]: wcs = WCS('2MASS_h.fits')

In [3]: wcs.wcs_pix2world([], [], 0)
---------------------------------------------------------------------------
InconsistentAxisTypesError                Traceback (most recent call last)
<ipython-input-3-e2cc0e97941a> in <module>()
----> 1 wcs.wcs_pix2world([], [], 0)

~/Dropbox/Code/Astropy/astropy/astropy/wcs/wcs.py in wcs_pix2world(self, *args, **kwargs)
   1352         return self._array_converter(
   1353             lambda xy, o: self.wcs.p2s(xy, o)['world'],
-> 1354             'output', *args, **kwargs)
   1355     wcs_pix2world.__doc__ = """
   1356         Transforms pixel coordinates to world coordinates by doing

~/Dropbox/Code/Astropy/astropy/astropy/wcs/wcs.py in _array_converter(self, func, sky, ra_dec_order, *args)
   1267                     "a 1-D array for each axis, followed by an origin.")
   1268 
-> 1269             return _return_list_of_arrays(axes, origin)
   1270 
   1271         raise TypeError(

~/Dropbox/Code/Astropy/astropy/astropy/wcs/wcs.py in _return_list_of_arrays(axes, origin)
   1223             if ra_dec_order and sky == 'input':
   1224                 xy = self._denormalize_sky(xy)
-> 1225             output = func(xy, origin)
   1226             if ra_dec_order and sky == 'output':
   1227                 output = self._normalize_sky(output)

~/Dropbox/Code/Astropy/astropy/astropy/wcs/wcs.py in <lambda>(xy, o)
   1351             raise ValueError("No basic WCS settings were created.")
   1352         return self._array_converter(
-> 1353             lambda xy, o: self.wcs.p2s(xy, o)['world'],
   1354             'output', *args, **kwargs)
   1355     wcs_pix2world.__doc__ = """

InconsistentAxisTypesError: ERROR 4 in wcsp2s() at line 2646 of file cextern/wcslib/C/wcs.c:
ncoord and/or nelem inconsistent with the wcsprm.
```

Match evidence

Primary terms

InconsistentAxisTypesErrorwcs_pix2worldastropy/wcs/wcs.pyastropy/astropyIssue when passing empty lists/arrays to WCS transformationsastropy.wcsWCS_array_converter_return_list_of_arraysself.wcs.p2swcsprmcextern/wcslib/C/wcs.c

Technical objective

Make the astropy.wcs.WCS coordinate-transformation entry points tolerate zero-length coordinate input. The deliverable is a change to the WCS transformation code path such that calling a transform with empty coordinate arrays returns empty results of the appropriate shape and type instead of propagating a wcslib error.

Success condition

a call of the form wcs.wcs_pix2world([], [], 0) (and the equivalent shapes on the other public transform wrappers) completes and yields empty output, while transformations of non-empty coordinates retain their current results, dtypes and error behaviour for genuinely malformed input.

Search fingerprints

  • Error signature: InconsistentAxisTypesError
  • Traceback path: astropy/wcs/wcs.py
  • Repository: astropy/astropy
  • Issue title: Issue when passing empty lists/arrays to WCS transformations.
  • Package/symbols: astropy.wcs, WCS, wcs_pix2world, _array_converter, _return_list_of_arrays, self.wcs.p2s, wcsprm.
  • Files: astropy/wcs/wcs.py, cextern/wcslib/C/wcs.c.
  • Exception: InconsistentAxisTypesError.
  • Verbatim error strings: ERROR 4 in wcsp2s() at line 2646 of file cextern/wcslib/C/wcs.c, ncoord and/or nelem inconsistent with the wcsprm.
  • Reproduction snippet: wcs.wcs_pix2world([], [], 0); data file 2MASS_h.fits.
  • Technologies: WCS / World Coordinate System, wcslib, FITS, IPython.

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