FIX: Restore access to private attr Nifti1Extension._content

nipygh-1336 reused the private attribute ``ext._content`` to exclusively
refer to the ``bytes`` representation of the extension contents.
This neglected that subclasses might depend on this implementation
detail.

Let's be nice to people and rename the attribute to ``_raw`` and provide
a ``_content`` property that calls ``self.get_content()``.

Also adds a test to ensure that multiple accesses continue to work as
expected.

nibabel/nifti1.py

@@ -326,7 +326,7 @@ class NiftiExtension(ty.Generic[T]):
 
     code: int
     encoding: str | None = None
-    _content: bytes
+    _raw: bytes
     _object: T | None = None
 
     def __init__(
@@ -351,10 +351,14 @@ def __init__(
             self.code = extension_codes.code[code]  # type: ignore[assignment]
         except KeyError:
             self.code = code  # type: ignore[assignment]
-        self._content = content
+        self._raw = content
         if object is not None:
             self._object = object
 
+    @property
+    def _content(self):
+        return self.get_object()
+
     @classmethod
     def from_bytes(cls, content: bytes) -> Self:
         """Create an extension from raw bytes.
@@ -394,15 +398,15 @@ def _sync(self) -> None:
         and updates the bytes representation accordingly.
         """
         if self._object is not None:
-            self._content = self._mangle(self._object)
+            self._raw = self._mangle(self._object)
 
     def __repr__(self) -> str:
         try:
             code = extension_codes.label[self.code]
         except KeyError:
             # deal with unknown codes
             code = self.code
-        return f'{self.__class__.__name__}({code}, {self._content!r})'
+        return f'{self.__class__.__name__}({code}, {self._raw!r})'
 
     def __eq__(self, other: object) -> bool:
         return (
@@ -425,7 +429,7 @@ def get_code(self):
     def content(self) -> bytes:
         """Return the extension content as raw bytes."""
         self._sync()
-        return self._content
+        return self._raw
 
     @property
     def text(self) -> str:
@@ -452,7 +456,7 @@ def get_object(self) -> T:
         instead.
         """
         if self._object is None:
-            self._object = self._unmangle(self._content)
+            self._object = self._unmangle(self._raw)
         return self._object
 
     # Backwards compatibility
@@ -488,7 +492,7 @@ def write_to(self, fileobj: ty.BinaryIO, byteswap: bool = False) -> None:
             extinfo = extinfo.byteswap()
         fileobj.write(extinfo.tobytes())
         # followed by the actual extension content, synced above
-        fileobj.write(self._content)
+        fileobj.write(self._raw)
         # be nice and zero out remaining part of the extension till the
         # next 16 byte border
         pad = extstart + rawsize - fileobj.tell()

nibabel/tests/test_nifti1.py

@@ -1250,6 +1250,33 @@ def test_extension_content_access():
     assert json_ext.json() == {'a': 1}
 
 
+def test_legacy_underscore_content():
+    """Verify that subclasses that depended on access to ._content continue to work."""
+    import io
+    import json
+
+    class MyLegacyExtension(Nifti1Extension):
+        def _mangle(self, value):
+            return json.dumps(value).encode()
+
+        def _unmangle(self, value):
+            if isinstance(value, bytes):
+                value = value.decode()
+            return json.loads(value)
+
+    ext = MyLegacyExtension(0, '{}')
+
+    assert isinstance(ext._content, dict)
+    # Object identity is not broken by multiple accesses
+    assert ext._content is ext._content
+
+    ext._content['val'] = 1
+
+    fobj = io.BytesIO()
+    ext.write_to(fobj)
+    assert fobj.getvalue() == b'\x20\x00\x00\x00\x00\x00\x00\x00{"val": 1}' + bytes(14)
+
+
 def test_extension_codes():
     for k in extension_codes.keys():
         Nifti1Extension(k, 'somevalue')