Fix importable class instances fail to parse and serialize (#448)

CHANGELOG.rst

@@ -12,6 +12,15 @@ The semantic versioning only considers the public API as described in
 paths are considered internals and can change in minor and patch releases.
 
 
+v4.27.5 (2024-02-??)
+--------------------
+
+Fixed
+^^^^^
+- Importable class instances fail to parse and serialize (`#446
+  <https://github.com/omni-us/jsonargparse/issues/446>`__).
+
+
 v4.27.4 (2024-02-01)
 --------------------
 

jsonargparse/_typehints.py

@@ -888,6 +888,8 @@ def adapt_typehints(
 
         try:
             val_class = import_object(resolve_class_path_by_name(typehint, val["class_path"]))
+            if isinstance(val_class, typehint):
+                return val_class
             if not is_subclass(val_class, typehint):
                 raise_unexpected_value(
                     f'Import path {val["class_path"]} does not correspond to a subclass of {typehint}'
@@ -1242,6 +1244,10 @@ def typehint_metavar(typehint):
 
 
 def serialize_class_instance(val):
+    with suppress(Exception):
+        import_path = get_import_path(val)
+        if import_path and import_object(import_path) is val:
+            return import_path
     val = f"Unable to serialize instance {val}"
     warning(val)
     return val

jsonargparse_tests/test_subclasses.py

@@ -328,6 +328,24 @@ def test_subclass_in_subcommand_with_global_default_config_file(parser, subparse
     assert cfg.fit.model.foo == 123
 
 
+# importable instances
+
+
+class dtype:
+    pass
+
+
+float32 = dtype()
+
+
+def test_importable_instances(parser):
+    parser.add_argument("--dtype", type=dtype)
+    cfg = parser.parse_args([f"--dtype={__name__}.float32"])
+    assert cfg.dtype is float32
+    dump = parser.dump(cfg)
+    assert dump.strip() == f"dtype: {__name__}.float32"
+
+
 # custom instantiation tests