Fix bug for defaults set to dataclass types

dcargs/_fields.py

@@ -530,7 +530,7 @@ def _ensure_dataclass_instance_used_as_default_is_frozen(
     frozen."""
     assert dataclasses.is_dataclass(default_instance)
     cls = type(default_instance)
-    if not cls.__dataclass_params__.frozen:
+    if not cls.__dataclass_params__.frozen:  # type: ignore
         warnings.warn(
             f"Mutable type {cls} is used as a default value for `{field.name}`. This is"
             " dangerous! Consider using `dataclasses.field(default_factory=...)` or"
@@ -566,7 +566,9 @@ def _get_dataclass_field_default(
     # Try grabbing default from dataclass field.
     if field.default not in MISSING_SINGLETONS:
         default = field.default
-        if dataclasses.is_dataclass(default):
+        # Note that dataclasses.is_dataclass() will also return true for dataclass
+        # _types_, not just instances.
+        if type(default) is not type and dataclasses.is_dataclass(default):
             _ensure_dataclass_instance_used_as_default_is_frozen(field, default)
         return default
 

tests/test_dcargs.py

@@ -468,6 +468,19 @@ def main(x: Callable[[int], int] = lambda x: x * 2) -> Callable[[int], int]:
         dcargs.cli(main, args=["--x", "something"])
 
 
+def test_fixed_dataclass_type():
+    @dataclasses.dataclass
+    class Dummy:
+        pass
+
+    def main(x: Callable = Dummy) -> Callable:
+        return x
+
+    assert dcargs.cli(main, args=[]) is Dummy
+    with pytest.raises(SystemExit):
+        dcargs.cli(main, args=["--x", "something"])
+
+
 def test_missing_singleton():
     assert dcargs.MISSING is copy.deepcopy(dcargs.MISSING)