DOC: update docstrings for ignore_blank, writeto()

astropy/io/fits/hdu/base.py

@@ -374,7 +374,7 @@ def readfrom(cls, fileobj, checksum=False, ignore_missing_end=False, **kwargs):
     def writeto(self, name, output_verify="exception", overwrite=False, checksum=False):
         """
         Write the HDU to a new file. This is a convenience method to
-        provide a user easier output interface if only one HDU needs
+        provide a user friendly output interface if only one HDU needs
         to be written to a file.
 
         Parameters
@@ -1600,9 +1600,9 @@ def match_header(cls, header):
 
     def writeto(self, name, output_verify="exception", overwrite=False, checksum=False):
         """
-        Works similarly to the normal writeto(), but prepends a default
-        `PrimaryHDU` are required by extension HDUs (which cannot stand on
-        their own).
+        Works similarly to :func:`_BaseHDU.writeto`, but prepends a default
+        `PrimaryHDU` as required by extension HDUs (which cannot be written
+        on their own).
         """
         from .hdulist import HDUList
         from .image import PrimaryHDU

astropy/io/fits/hdu/image.py

@@ -779,9 +779,9 @@ def _convert_pseudo_integer(self, data):
         Handle "pseudo-unsigned" integers, if the user requested it.  Returns
         the converted data array if so; otherwise returns None.
 
-        In this case case, we don't need to handle BLANK to convert it to NAN,
-        since we can't do NaNs with integers, anyway, i.e. the user is
-        responsible for managing blanks.
+        Handling of BLANK is done outside of this function on the converted
+        uint data, meaning that BLANK needs to be shifted by BZERO as well
+        before conversion of the corresponding values to NaN.
         """
         dtype = self._dtype_for_bitpix()
         # bool(dtype) is always False--have to explicitly compare to None; this

astropy/io/fits/tests/test_image.py

@@ -757,7 +757,7 @@ def test_uint_header_keywords_removed_after_bitpix_change(
     @pytest.mark.parametrize(("ignore_blank"), (False, True))
     def test_blanks(self, ext, ignore_blank):
         """Test image data with blank spots in it (which should show up as
-        NaNs in the data array, unless 'ignore_blank' is set).
+        NaNs in the data array, unless ignore_blank is set to True).
         """
 
         arr = np.zeros((10, 10), dtype=np.int32)
@@ -768,12 +768,12 @@ def test_blanks(self, ext, ignore_blank):
             hdu = fits.PrimaryHDU(data=arr)
         else:
             hdu = fits.ImageHDU(data=arr)
-        hdu.header["BLANK"] = 999
+        hdu.header["BLANK"] = 99999
         hdu.writeto(self.temp("test_blank.fits"))
 
         with fits.open(self.temp("test_blank.fits"), ignore_blank=ignore_blank) as hdul:
             if ignore_blank:
-                assert hdul[ext].data.dtype == '>i4'
+                assert hdul[ext].data.dtype == ">i4"
                 assert (hdul[ext].data[1] == hdu.header["BLANK"]).all()
             else:
                 assert hdul[ext].data.dtype == np.float64
@@ -783,7 +783,7 @@ def test_blanks(self, ext, ignore_blank):
     @pytest.mark.parametrize(("ignore_blank"), (False, True))
     def test_uint_blanks(self, ext, ignore_blank):
         """Test uint image data with blank spots in it (should be controlled
-        by 'ignore_blank' in the same way as for unscaled integers).
+        by ignore_blank in the same way as for unscaled integers).
         """
 
         arr = np.arange(100, dtype=np.uint16).reshape((10, 10))

docs/changes/io.fits/16636.bugfix.rst

@@ -0,0 +1,4 @@
+Fix handling of ``ignore_blank`` keyword for uncompressed integer data.
+Set to True, will keep original integer data with 'BLANK' values,
+to False, convert to float with NaNs, for both signed and unsigned
+integers, and primary as well as extension HDUs.