add new FITSFormatError and raise for missing XTENSION

can add more cases over time

CHANGES.md

@@ -1,3 +1,13 @@
+version 1.2.1 (not yet released)
+-------------
+
+Changes
+
+    - raise new fitsio.FITSFormatError exception when we detect format
+      errors in a FITS file.  Currently this only raises when extensions
+      are not properly marked with XTENSION but we can expand this over time
+
+
 version 1.2.0
 --------------
 

fitsio/__init__.py

@@ -35,3 +35,5 @@
 
 from . import util
 from .util import cfitsio_version, FITSRuntimeWarning
+
+from .fits_exceptions import FITSFormatError

fitsio/fits_exceptions.py

@@ -0,0 +1,10 @@
+class FITSFormatError(Exception):
+    """
+    Format error in FITS file
+    """
+    def __init__(self, value):
+        super(FITSFormatError, self).__init__(value)
+        self.value = value
+
+    def __str__(self):
+        return str(self.value)

fitsio/fitslib.py

@@ -32,6 +32,8 @@
     ImageHDU, AsciiTableHDU, TableHDU,
     _table_npy2fits_form, _npy2fits, _hdu_type_map)
 
+from .fits_exceptions import FITSFormatError
+
 # for python3 compat
 if IS_PY3:
     xrange = range
@@ -540,15 +542,33 @@ def movabs_ext(self, ext):
 
         In general, it is not necessary to use this method explicitly.
         """
-        return self._FITS.movabs_hdu(ext+1)
+        return self.movabs_hdu(ext+1)
 
     def movabs_hdu(self, hdunum):
         """
         Move to the indicated one-offset hdu number.
 
         In general, it is not necessary to use this method explicitly.
         """
-        return self._FITS.movabs_hdu(hdunum)
+
+        format_err = False
+
+        try:
+            hdu_type = self._FITS.movabs_hdu(hdunum)
+        except IOError as err:
+            # to support python 2 we can't use exception chaining.
+            # do this to avoid "During handling of the above exception, another
+            # exception occurred:"
+            serr = str(err)
+            if 'first keyword not XTENSION' in serr:
+                format_err = True
+            else:
+                raise
+
+        if format_err:
+            raise FITSFormatError(serr)
+
+        return hdu_type
 
     def movnam_ext(self, extname, hdutype=ANY_HDU, extver=0):
         """
@@ -570,8 +590,24 @@ def movnam_hdu(self, extname, hdutype=ANY_HDU, extver=0):
 
         returns the one-offset extension number
         """
+        format_err = False
+
         extname = mks(extname)
-        hdu = self._FITS.movnam_hdu(hdutype, extname, extver)
+        try:
+            hdu = self._FITS.movnam_hdu(hdutype, extname, extver)
+        except IOError as err:
+            # to support python 2 we can't use exception chaining.
+            # do this to avoid "During handling of the above exception, another
+            # exception occurred:"
+            serr = str(err)
+            if 'first keyword not XTENSION' in serr:
+                format_err = True
+            else:
+                raise
+
+        if format_err:
+            raise FITSFormatError(serr)
+
         return hdu
 
     def reopen(self):
@@ -1302,7 +1338,7 @@ def _append_hdu_info(self, ext):
         """
 
         # raised IOError if not found
-        hdu_type = self._FITS.movabs_hdu(ext+1)
+        hdu_type = self.movabs_ext(ext)
 
         if hdu_type == IMAGE_HDU:
             hdu = ImageHDU(self._FITS, ext)

fitsio/tests/test_header_junk.py

@@ -1,6 +1,8 @@
 import os
 import tempfile
-from ..fitslib import read_header
+import pytest
+from ..fitslib import read_header, FITS
+from ..fits_exceptions import FITSFormatError
 
 
 def test_header_junk():
@@ -43,3 +45,22 @@ def test_Header_Junk_Non_Ascii():
 
         h = read_header(fname)
         assert h["@_@_"] is None
+
+
+def test_missing_xtension_keyword():
+    """
+    Misformatted header with extension not properly marked with
+    XTENSION
+    """
+
+    data=b"""SIMPLE  =                    T / This is a FITS file                            BITPIX  =                    8 /                                                NAXIS   =                    0 /                                                EXTEND  =                    T / This file may contain FITS extensions          NEXTEND =                    7 / Number of extensions                           END                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             SIMPLE  =                    T / file does conform to FITS standard             BITPIX  =                   32 / number of bits per data pixel                  NAXIS   =                    2 / number of data axes                            NAXIS1  =                   30 / length of data axis 1                          NAXIS2  =                   30 / length of data axis 2                          EXTEND  =                    T / FITS dataset may contain extensions            END                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             """  # noqa
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        fname = os.path.join(tmpdir, 'test.fits')
+
+        with open(fname, 'wb') as fobj:
+            fobj.write(data)
+
+        with pytest.raises(FITSFormatError):
+            with FITS(fname) as fits:
+                print(fits)