Backport PR astropy#15001: Fixed a bug that caused compressed images …

…with TFORM missing the optional '1' prefix to not be readable

astropy/io/fits/_tiled_compression/tiled_compression.py

@@ -203,12 +203,13 @@ def _check_compressed_header(header):
             if not np.isreal(header[kw]) or not float(header[kw]).is_integer():
                 raise TypeError(f"{kw} should be an integer")
 
-    if "TFORM1" in header:
-        for valid in ["1PB", "1PI", "1PJ", "1QB", "1QI", "1QJ"]:
-            if header["TFORM1"].startswith(valid):
-                break
-        else:
-            raise RuntimeError(f"Invalid TFORM1: {header['TFORM1']}")
+    for suffix in range(1, header["TFIELDS"] + 1):
+        if header.get(f"TTYPE{suffix}", "").endswith("COMPRESSED_DATA"):
+            for valid in ["PB", "PI", "PJ", "QB", "QI", "QJ"]:
+                if header[f"TFORM{suffix}"].startswith((valid, f"1{valid}")):
+                    break
+            else:
+                raise RuntimeError(f"Invalid TFORM{suffix}: {header[f'TFORM{suffix}']}")
 
     # Check values
 
@@ -253,11 +254,13 @@ def _get_compression_setting(header, name, default):
 
 def _column_dtype(hdu, column_name):
     tform = hdu.columns[column_name].format
+    if tform.startswith("1"):
+        tform = tform[1:]
     if tform[2] == "B":
         dtype = np.uint8
-    elif tform[2] == "I":
+    elif tform[1] == "I":
         dtype = ">i2"
-    elif tform[2] == "J":
+    elif tform[1] == "J":
         dtype = ">i4"
     return np.dtype(dtype)
 

astropy/io/fits/tests/test_image.py

@@ -2009,6 +2009,32 @@ def test_comp_image_properties_set(self):
         assert chdu.tile_shape == (2, 3, 4)
         assert chdu.compression_type == "PLIO_1"
 
+    def test_compressed_optional_prefix_tform(self, tmp_path):
+        # Regression test for a bug that caused an error if a
+        # compressed file had TFORM missing the optional 1 prefix
+
+        data = np.zeros((3, 4, 5))
+
+        hdu1 = fits.CompImageHDU(data=data)
+        hdu1.writeto(tmp_path / "compressed.fits")
+
+        with fits.open(
+            tmp_path / "compressed.fits", disable_image_compression=True, mode="update"
+        ) as hdul:
+            assert hdul[1].header["TFORM1"] == "1PB(0)"
+            assert hdul[1].header["TFORM2"] == "1PB(24)"
+            hdul[1].header["TFORM1"] = "PB(0)"
+            hdul[1].header["TFORM2"] = "PB(24)"
+
+        with fits.open(
+            tmp_path / "compressed.fits", disable_image_compression=True
+        ) as hdul:
+            assert hdul[1].header["TFORM1"] == "PB(0)"
+            assert hdul[1].header["TFORM2"] == "PB(24)"
+
+        with fits.open(tmp_path / "compressed.fits") as hdul:
+            assert_equal(hdul[1].data, data)
+
 
 class TestCompHDUSections:
     @pytest.fixture(autouse=True)

docs/changes/io.fits/15001.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug that caused compressed images with TFORM missing the optional '1' prefix to not be readable.