BUG: Fix isna cannot handle ambiguous typed list (#20971)

doc/source/whatsnew/v0.23.0.txt

@@ -1365,6 +1365,7 @@ Reshaping
 - Bug in :func:`cut` and :func:`qcut` where timezone information was dropped (:issue:`19872`)
 - Bug in :class:`Series` constructor with a ``dtype=str``, previously raised in some cases (:issue:`19853`)
 - Bug in :func:`get_dummies`, and :func:`select_dtypes`, where duplicate column names caused incorrect behavior (:issue:`20848`)
+- Bug in :func:`isna`, which cannot handle ambiguous typed lists (:issue:`20675`)
 
 Other
 ^^^^^

pandas/core/dtypes/missing.py

@@ -120,7 +120,9 @@ def _isna_new(obj):
         return _isna_ndarraylike(obj)
     elif isinstance(obj, ABCGeneric):
         return obj._constructor(obj._data.isna(func=isna))
-    elif isinstance(obj, list) or hasattr(obj, '__array__'):
+    elif isinstance(obj, list):
+        return _isna_ndarraylike(np.asarray(obj, dtype=object))
+    elif hasattr(obj, '__array__'):
         return _isna_ndarraylike(np.asarray(obj))
     else:
         return obj is None
@@ -146,7 +148,9 @@ def _isna_old(obj):
         return _isna_ndarraylike_old(obj)
     elif isinstance(obj, ABCGeneric):
         return obj._constructor(obj._data.isna(func=_isna_old))
-    elif isinstance(obj, list) or hasattr(obj, '__array__'):
+    elif isinstance(obj, list):
+        return _isna_ndarraylike_old(np.asarray(obj, dtype=object))
+    elif hasattr(obj, '__array__'):
         return _isna_ndarraylike_old(np.asarray(obj))
     else:
         return obj is None

pandas/tests/dtypes/test_missing.py

@@ -118,6 +118,11 @@ def test_isna_lists(self):
         exp = np.array([False, False])
         tm.assert_numpy_array_equal(result, exp)
 
+        # GH20675
+        result = isna([np.NaN, 'world'])
+        exp = np.array([True, False])
+        tm.assert_numpy_array_equal(result, exp)
+
     def test_isna_nat(self):
         result = isna([NaT])
         exp = np.array([True])