Fix expanding home directory for Trainer's default_root_dir (#19179)

Author: asingh9530

src/lightning/pytorch/CHANGELOG.md

@@ -83,6 +83,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed issue where the `precision="transformer-engine"` argument would not replace layers by default ([#19082](https://github.com/Lightning-AI/lightning/pull/19082))
 
 
+- Fixed `Trainer` not expanding the `default_root_dir` if it has the `~` (home) prefix ([#19179](https://github.com/Lightning-AI/lightning/pull/19179))
+
+
 ## [2.1.2] - 2023-11-15
 
 ### Fixed

src/lightning/pytorch/trainer/trainer.py

@@ -1285,7 +1285,7 @@ def default_root_dir(self) -> str:
 
         """
         if _is_local_file_protocol(self._default_root_dir):
-            return os.path.normpath(self._default_root_dir)
+            return os.path.normpath(os.path.expanduser(self._default_root_dir))
         return self._default_root_dir
 
     @property

tests/tests_pytorch/trainer/test_trainer.py

@@ -2064,3 +2064,13 @@ def test_init_module_context(monkeypatch):
     with pytest.warns(PossibleUserWarning, match="can't place .* on the device"), trainer.init_module():
         pass
     strategy.tensor_init_context.assert_called_once()
+
+
+def test_expand_home_trainer():
+    """Test that the dirpath gets expanded if it contains `~`."""
+    home_root = Path.home()
+
+    trainer = Trainer(default_root_dir="~/trainer")
+    assert trainer.default_root_dir == str(home_root / "trainer")
+    trainer = Trainer(default_root_dir=Path("~/trainer"))
+    assert trainer.default_root_dir == str(home_root / "trainer")