RF: go back to using decorator for .mgz extension

At Ben's suggestion, restore the decorator for now, to simplify
discussion of this PR.

Author: matthew-brett

nibabel/freesurfer/mghformat.py

@@ -455,14 +455,12 @@ def writeftr_to(self, fileobj):
 
 
 # Register .mgz extension as compressed
-ImageOpener.compress_ext_map['.mgz'] = ImageOpener.gz_def
-
-
+@ImageOpener.register_ext_from_image('.mgz', ImageOpener.gz_def)
 class MGHImage(SpatialImage):
     """ Class for MGH format image
     """
     header_class = MGHHeader
-    valid_exts = ('.mgh', '.mgz')
+    valid_exts = ('.mgh',)
     files_types = (('image', '.mgh'),)
     _compressed_suffixes = ()
 

nibabel/openers.py

@@ -185,6 +185,6 @@ def decorate(klass):
             assert ext not in opener_klass.compress_ext_map, \
                 "Cannot redefine extension-function mappings."
             opener_klass.compress_ext_map[ext] = func_def
-            klass.alternate_exts += (ext,)
+            klass.valid_exts += (ext,)
             return klass
         return decorate

nibabel/tests/test_openers.py

@@ -93,7 +93,8 @@ def test_BinOpener():
 
 
 class TestImageOpener:
-    alternate_exts = ()
+    valid_exts = ()
+
     def setUp(self):
         self.compress_ext_map = ImageOpener.compress_ext_map.copy()
 
@@ -119,7 +120,7 @@ def file_opener(fileish, mode):
         dec(self.__class__)
         assert_equal(n_associations + 1, len(ImageOpener.compress_ext_map))
         assert_true('.foo' in ImageOpener.compress_ext_map)
-        assert_true('.foo' in self.alternate_exts)
+        assert_true('.foo' in self.valid_exts)
 
         with InTemporaryDirectory():
             with ImageOpener('test.foo', 'w'):