Support index diffs against the empty tree

git/index/base.py

@@ -1480,12 +1480,11 @@ def reset(
 
         return self
 
-    # FIXME: This is documented to accept the same parameters as Diffable.diff, but this
-    # does not handle NULL_TREE for `other`. (The suppressed mypy error is about this.)
     def diff(
         self,
-        other: Union[  # type: ignore[override]
+        other: Union[
             Literal[git_diff.DiffConstants.INDEX],
+            Literal[git_diff.DiffConstants.NULL_TREE],
             "Tree",
             "Commit",
             str,
@@ -1512,6 +1511,44 @@ def diff(
         if other is self.INDEX:
             return git_diff.DiffIndex()
 
+        if other is git_diff.NULL_TREE:
+            args: List[Union[PathLike, str]] = [
+                "--cached",
+                "4b825dc642cb6eb9a060e54bf8d69288fbee4904",
+                "--abbrev=40",
+                "--full-index",
+            ]
+
+            if not any(x in kwargs for x in ("find_renames", "no_renames", "M")):
+                args.append("-M")
+
+            if create_patch:
+                args.append("-p")
+                args.append("--no-ext-diff")
+            else:
+                args.append("--raw")
+                args.append("-z")
+
+            args.append("--no-color")
+
+            if paths is not None and not isinstance(paths, (tuple, list)):
+                paths = [paths]
+
+            if paths:
+                args.append("--")
+                args.extend(paths)
+
+            kwargs["as_process"] = True
+            proc = self.repo.git.diff(*args, **kwargs)
+
+            diff_method = (
+                git_diff.Diff._index_from_patch_format if create_patch else git_diff.Diff._index_from_raw_format
+            )
+            index = diff_method(self.repo, proc)
+
+            proc.wait()
+            return index
+
         # Index against anything but None is a reverse diff with the respective item.
         # Handle existing -R flags properly.
         # Transform strings to the object so that we can call diff on it.

test/test_index.py

@@ -23,7 +23,7 @@
 import ddt
 import pytest
 
-from git import BlobFilter, Diff, Git, IndexFile, Object, Repo, Tree
+from git import BlobFilter, Diff, Git, IndexFile, NULL_TREE, Object, Repo, Tree
 from git.exc import (
     CheckoutError,
     GitCommandError,
@@ -555,6 +555,34 @@ def test_index_file_diffing(self, rw_repo):
         rval = index.checkout("lib")
         assert len(list(rval)) > 1
 
+    @with_rw_directory
+    def test_index_file_diff_null_tree_with_initial_index(self, rw_dir):
+        repo = Repo.init(rw_dir)
+        filename = ".gitkeep"
+        file_path = osp.join(repo.working_tree_dir, filename)
+        with open(file_path, "w") as fp:
+            fp.write("# Initial file\n")
+
+        index = repo.index
+        index.add([filename])
+        index.write()
+
+        index = IndexFile(repo)
+        assert not index.diff(None)
+
+        diff = index.diff(NULL_TREE)
+        self.assertEqual(len(diff), 1)
+        self.assertEqual(diff[0].change_type, "A")
+        assert diff[0].new_file
+        self.assertEqual(diff[0].b_path, filename)
+
+        self.assertEqual(len(index.diff(NULL_TREE, paths=filename)), 1)
+        self.assertEqual(len(index.diff(NULL_TREE, paths="missing")), 0)
+
+        patch = index.diff(NULL_TREE, create_patch=True)
+        self.assertEqual(len(patch), 1)
+        self.assertIn(b"+# Initial file", patch[0].diff)
+
     def _count_existing(self, repo, files):
         """Return count of files that actually exist in the repository directory."""
         existing = 0