Fix ModelCheckpoint dirpath expanding home prefix (#19058)

(cherry picked from commit 58c905b)

src/lightning/pytorch/CHANGELOG.md

@@ -23,6 +23,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue where setting `CHECKPOINT_JOIN_CHAR` or `CHECKPOINT_EQUALS_CHAR` would only work on the `ModelCheckpoint` class but not on an instance ([#19054](https://github.com/Lightning-AI/lightning/pull/19054))
 
 
+- Fixed `ModelCheckpoint` not expanding the `dirpath` if it has the `~` (home) prefix ([#19058](https://github.com/Lightning-AI/lightning/pull/19058))
+
+
+
 ## [2.1.2] - 2023-11-15
 
 ### Fixed

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -458,7 +458,7 @@ def __init_ckpt_dir(self, dirpath: Optional[_PATH], filename: Optional[str]) ->
         self._fs = get_filesystem(dirpath if dirpath else "")
 
         if dirpath and _is_local_file_protocol(dirpath if dirpath else ""):
-            dirpath = os.path.realpath(dirpath)
+            dirpath = os.path.realpath(os.path.expanduser(dirpath))
 
         self.dirpath = dirpath
         self.filename = filename

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -1536,3 +1536,17 @@ def test_find_last_checkpoints(name, extension, folder_contents, expected, tmp_p
     callback.FILE_EXTENSION = extension
     files = callback._find_last_checkpoints(trainer)
     assert files == {str(tmp_path / p) for p in expected}
+
+
+def test_expand_home():
+    """Test that the dirpath gets expanded if it contains `~`."""
+    home_root = Path.home()
+
+    checkpoint = ModelCheckpoint(dirpath="~/checkpoints")
+    assert checkpoint.dirpath == str(home_root / "checkpoints")
+    checkpoint = ModelCheckpoint(dirpath=Path("~/checkpoints"))
+    assert checkpoint.dirpath == str(home_root / "checkpoints")
+
+    # it is possible to have a folder with the name `~`
+    checkpoint = ModelCheckpoint(dirpath="./~/checkpoints")
+    assert checkpoint.dirpath == str(Path.cwd() / "~" / "checkpoints")