Always use a def instead of assigning a lambda

This stops listing Ruff rule `E731` ("Do not assign a `lambda`
expression, use a `def`") as ignored, and fixes all occurrences of
it:

- Spacing is manually adjusted so that readability is not harmed,
  while still satisfying the current formatting conventions.

- Although the affected test modules do not currently use type
  annotations, the non-test modules do. Some of the lambdas already
  had type annotations, by annotating the variable itself with an
  expression formed by subscripting `Callable`. This change
  preserves them, converting them to paramter and return type
  annotations in the resulting `def`. Where such type annotations
  were absent (in lambdas in non-test modules), or partly absent,
  all missing annotations are added to the `def`.

- Unused paramters are prefixed with a `_`.

- `IndexFile.checkout` assigned a lambda to `make_exc`, whose body
  was somewhat difficult to read. Separately from converting it to
  a `def`, this refactors the expression in the `return` statement
  to use code like `(x, *ys)` in place of `(x,) + tuple(ys)`.

This change does not appear to have introduced (nor fixed) any
`mypy` errors.

This only affects lambdas that were assigned directly to variables.
Other lambda expressions remain unchanged.

git/index/base.py

@@ -530,7 +530,10 @@ def unmerged_blobs(self) -> Dict[PathLike, List[Tuple[StageType, Blob]]]:
             stage. That is, a file removed on the 'other' branch whose entries are at
             stage 3 will not have a stage 3 entry.
         """
-        is_unmerged_blob = lambda t: t[0] != 0
+
+        def is_unmerged_blob(t: Tuple[StageType, Blob]) -> bool:
+            return t[0] != 0
+
         path_map: Dict[PathLike, List[Tuple[StageType, Blob]]] = {}
         for stage, blob in self.iter_blobs(is_unmerged_blob):
             path_map.setdefault(blob.path, []).append((stage, blob))
@@ -690,12 +693,17 @@ def _store_path(self, filepath: PathLike, fprogress: Callable) -> BaseIndexEntry
             This must be ensured in the calling code.
         """
         st = os.lstat(filepath)  # Handles non-symlinks as well.
+
         if S_ISLNK(st.st_mode):
             # In PY3, readlink is a string, but we need bytes.
             # In PY2, it was just OS encoded bytes, we assumed UTF-8.
-            open_stream: Callable[[], BinaryIO] = lambda: BytesIO(force_bytes(os.readlink(filepath), encoding=defenc))
+            def open_stream() -> BinaryIO:
+                return BytesIO(force_bytes(os.readlink(filepath), encoding=defenc))
         else:
-            open_stream = lambda: open(filepath, "rb")
+
+            def open_stream() -> BinaryIO:
+                return open(filepath, "rb")
+
         with open_stream() as stream:
             fprogress(filepath, False, filepath)
             istream = self.repo.odb.store(IStream(Blob.type, st.st_size, stream))
@@ -1336,8 +1344,11 @@ def handle_stderr(proc: "Popen[bytes]", iter_checked_out_files: Iterable[PathLik
             kwargs["as_process"] = True
             kwargs["istream"] = subprocess.PIPE
             proc = self.repo.git.checkout_index(args, **kwargs)
+
             # FIXME: Reading from GIL!
-            make_exc = lambda: GitCommandError(("git-checkout-index",) + tuple(args), 128, proc.stderr.read())
+            def make_exc() -> GitCommandError:
+                return GitCommandError(("git-checkout-index", *args), 128, proc.stderr.read())
+
             checked_out_files: List[PathLike] = []
 
             for path in paths:

git/objects/tree.py

@@ -50,7 +50,9 @@
 
 # --------------------------------------------------------
 
-cmp: Callable[[str, str], int] = lambda a, b: (a > b) - (a < b)
+
+def cmp(a: str, b: str) -> int:
+    return (a > b) - (a < b)
 
 
 class TreeModifier:

pyproject.toml

@@ -66,7 +66,7 @@ lint.extend-select = [
     "TC004",  # See: https://docs.astral.sh/ruff/rules/runtime-import-in-type-checking-block/
 ]
 lint.ignore = [
-    "E731",  # Do not assign a `lambda` expression, use a `def`
+    # If it becomes necessary to ignore any rules, list them here.
 ]
 lint.unfixable = [
     "F401",  # Module imported but unused

test/test_fun.py

@@ -243,6 +243,7 @@ def test_tree_traversal(self):
         B_old = self.rorepo.tree("1f66cfbbce58b4b552b041707a12d437cc5f400a")  # old base tree
 
         # Two very different trees.
+
         entries = traverse_trees_recursive(odb, [B_old.binsha, H.binsha], "")
         self._assert_tree_entries(entries, 2)
 
@@ -251,7 +252,10 @@ def test_tree_traversal(self):
         self._assert_tree_entries(oentries, 2)
 
         # Single tree.
-        is_no_tree = lambda i, d: i.type != "tree"
+
+        def is_no_tree(i, _d):
+            return i.type != "tree"
+
         entries = traverse_trees_recursive(odb, [B.binsha], "")
         assert len(entries) == len(list(B.traverse(predicate=is_no_tree)))
         self._assert_tree_entries(entries, 1)

test/test_index.py

@@ -330,7 +330,10 @@ def test_index_file_from_tree(self, rw_repo):
         assert len([e for e in three_way_index.entries.values() if e.stage != 0])
 
         # ITERATE BLOBS
-        merge_required = lambda t: t[0] != 0
+
+        def merge_required(t):
+            return t[0] != 0
+
         merge_blobs = list(three_way_index.iter_blobs(merge_required))
         assert merge_blobs
         assert merge_blobs[0][0] in (1, 2, 3)

test/test_tree.py

@@ -126,12 +126,18 @@ def test_traverse(self):
         assert len(list(root)) == len(list(root.traverse(depth=1)))
 
         # Only choose trees.
-        trees_only = lambda i, d: i.type == "tree"
+
+        def trees_only(i, _d):
+            return i.type == "tree"
+
         trees = list(root.traverse(predicate=trees_only))
         assert len(trees) == len([i for i in root.traverse() if trees_only(i, 0)])
 
         # Test prune.
-        lib_folder = lambda t, d: t.path == "lib"
+
+        def lib_folder(t, _d):
+            return t.path == "lib"
+
         pruned_trees = list(root.traverse(predicate=trees_only, prune=lib_folder))
         assert len(pruned_trees) < len(trees)