Merge pull request #6489 from MSeifert04/ccddata_fix_problem_with_hea…

…der_combine_reader

Only keep Header values from the Primary that are not present in extension.

CHANGES.rst

@@ -45,6 +45,10 @@ astropy.nddata
 
 - Suppress errors during WCS creation in CCDData.read(). [#6500]
 
+- Fixed a problem with ``CCDData.read`` when the extension wasn't given and the
+  primary HDU contained no ``data`` but another HDU did. In that case the header
+  were not correctly combined. [#6489]
+
 astropy.samp
 ^^^^^^^^^^^^
 

astropy/nddata/ccddata.py

@@ -504,7 +504,11 @@ def fits_ccddata_reader(filename, hdu=0, unit=None, hdu_uncertainty='UNCERT',
             for i in range(len(hdus)):
                 if hdus.fileinfo(i)['datSpan'] > 0:
                     hdu = i
-                    hdr = hdr + hdus[hdu].header
+                    comb_hdr = hdus[hdu].header.copy()
+                    # Add header values from the primary header that aren't
+                    # present in the extension header.
+                    comb_hdr.extend(hdr, unique=True)
+                    hdr = comb_hdr
                     log.info("first HDU with data is extension "
                              "{0}.".format(hdu))
                     break

astropy/nddata/tests/test_ccddata.py

@@ -149,17 +149,18 @@ def test_initialize_from_fits_with_ADU_in_header(tmpdir):
 
 def test_initialize_from_fits_with_data_in_different_extension(tmpdir):
     fake_img = np.random.random(size=(100, 100))
-    new_hdul = fits.HDUList()
     hdu1 = fits.PrimaryHDU()
     hdu2 = fits.ImageHDU(fake_img)
     hdus = fits.HDUList([hdu1, hdu2])
     filename = tmpdir.join('afile.fits').strpath
     hdus.writeto(filename)
-    ccd = CCDData.read(filename, unit='adu')
+    with catch_warnings(FITSFixedWarning) as w:
+        ccd = CCDData.read(filename, unit='adu')
+    assert len(w) == 0
     # ccd should pick up the unit adu from the fits header...did it?
     np.testing.assert_array_equal(ccd.data, fake_img)
     # check that the header is the combined header
-    assert hdu1.header + hdu2.header == ccd.header
+    assert hdu2.header + hdu1.header == ccd.header
 
 
 def test_initialize_from_fits_with_extension(tmpdir):