BUG: Fixed Series.align(frame) with ExtensionArray (#20580)

Closes #20576

Author: TomAugspurger

pandas/core/dtypes/cast.py

@@ -12,6 +12,7 @@
                      is_complex, is_datetimetz, is_categorical_dtype,
                      is_datetimelike,
                      is_extension_type,
+                     is_extension_array_dtype,
                      is_object_dtype,
                      is_datetime64tz_dtype, is_datetime64_dtype,
                      is_datetime64_ns_dtype,
@@ -329,7 +330,7 @@ def maybe_promote(dtype, fill_value=np.nan):
         dtype = np.object_
 
     # in case we have a string that looked like a number
-    if is_categorical_dtype(dtype):
+    if is_extension_array_dtype(dtype):
         pass
     elif is_datetimetz(dtype):
         pass

pandas/tests/extension/base/reshaping.py

@@ -1,4 +1,5 @@
 import pytest
+import numpy as np
 
 import pandas as pd
 from pandas.core.internals import ExtensionBlock
@@ -64,6 +65,19 @@ def test_align_frame(self, data, na_value):
         self.assert_frame_equal(r1, e1)
         self.assert_frame_equal(r2, e2)
 
+    def test_align_series_frame(self, data, na_value):
+        # https://github.com/pandas-dev/pandas/issues/20576
+        ser = pd.Series(data, name='a')
+        df = pd.DataFrame({"col": np.arange(len(ser) + 1)})
+        r1, r2 = ser.align(df)
+
+        e1 = pd.Series(
+            data._constructor_from_sequence(list(data) + [na_value]),
+            name=ser.name)
+
+        self.assert_series_equal(r1, e1)
+        self.assert_frame_equal(r2, df)
+
     def test_set_frame_expand_regular_with_extension(self, data):
         df = pd.DataFrame({"A": [1] * len(data)})
         df['B'] = data