BUG(io): store toc dtype when opening an existing FITS (#106)

Fixes a bug that meant the dtype of the toc of existing FITS files was
not stored.

Also adds a missing test case which would have shown the bug earlier.

Fixes: #105

heracles/io.py

@@ -788,7 +788,7 @@ def __init__(self, path, *, clobber=False):
                 toc = fits[self.ext].read()
 
                 # store the dtype for ToC entries
-                toc.dtype = toc.dtype
+                self.dtype = toc.dtype
 
                 # store the ToC as a mapping
                 self._toc = TocDict({tuple(key): str(ext) for ext, *key in toc})

tests/test_io.py

@@ -358,6 +358,7 @@ class TestFits(TocFits):
 
     data12 = np.zeros(5, dtype=[("X", float), ("Y", int)])
     data22 = np.ones(5, dtype=[("X", float), ("Y", int)])
+    data21 = np.full(5, 2, dtype=[("X", float), ("Y", int)])
 
     tocfits[1, 2] = data12
 
@@ -400,6 +401,16 @@ class TestFits(TocFits):
     np.testing.assert_array_equal(tocfits2[1, 2], data12)
     np.testing.assert_array_equal(tocfits2[2, 2], data22)
 
+    tocfits2[2, 1] = data21
+
+    with fitsio.FITS(path) as fits:
+        assert len(fits) == 5
+        toc = fits["TESTTOC"].read()
+        assert len(toc) == 3
+        np.testing.assert_array_equal(fits["TEST0"].read(), data12)
+        np.testing.assert_array_equal(fits["TEST1"].read(), data22)
+        np.testing.assert_array_equal(fits["TEST2"].read(), data21)
+
 
 def test_tocfits_is_lazy(tmp_path):
     import fitsio