BUG: ExtensionArray.fillna for scalar values (#20412)

Closes #20411

Author: TomAugspurger

pandas/core/arrays/base.py

@@ -261,15 +261,15 @@ def fillna(self, value=None, method=None, limit=None):
         -------
         filled : ExtensionArray with NA/NaN filled
         """
-        from pandas.api.types import is_scalar
+        from pandas.api.types import is_array_like
         from pandas.util._validators import validate_fillna_kwargs
         from pandas.core.missing import pad_1d, backfill_1d
 
         value, method = validate_fillna_kwargs(value, method)
 
         mask = self.isna()
 
-        if not is_scalar(value):
+        if is_array_like(value):
             if len(value) != len(self):
                 raise ValueError("Length of 'value' does not match. Got ({}) "
                                  " expected {}".format(len(value), len(self)))

pandas/tests/extension/base/base.py

@@ -4,3 +4,6 @@
 class BaseExtensionTests(object):
     assert_series_equal = staticmethod(tm.assert_series_equal)
     assert_frame_equal = staticmethod(tm.assert_frame_equal)
+    assert_extension_array_equal = staticmethod(
+        tm.assert_extension_array_equal
+    )

pandas/tests/extension/base/missing.py

@@ -47,6 +47,12 @@ def test_dropna_frame(self, data_missing):
         expected = df.iloc[:0]
         self.assert_frame_equal(result, expected)
 
+    def test_fillna_scalar(self, data_missing):
+        valid = data_missing[1]
+        result = data_missing.fillna(valid)
+        expected = data_missing.fillna(valid)
+        self.assert_extension_array_equal(result, expected)
+
     def test_fillna_limit_pad(self, data_missing):
         arr = data_missing.take([1, 0, 0, 0, 1])
         result = pd.Series(arr).fillna(method='ffill', limit=2)

pandas/util/testing.py

@@ -20,6 +20,7 @@
 import numpy as np
 
 import pandas as pd
+from pandas.core.arrays import ExtensionArray
 from pandas.core.dtypes.missing import array_equivalent
 from pandas.core.dtypes.common import (
     is_datetimelike_v_numeric,
@@ -1083,6 +1084,32 @@ def _raise(left, right, err_msg):
     return True
 
 
+def assert_extension_array_equal(left, right):
+    """Check that left and right ExtensionArrays are equal.
+
+    Parameters
+    ----------
+    left, right : ExtensionArray
+        The two arrays to compare
+
+    Notes
+    -----
+    Missing values are checked separately from valid values.
+    A mask of missing values is computed for each and checked to match.
+    The remaining all-valid values are cast to object dtype and checked.
+    """
+    assert isinstance(left, ExtensionArray)
+    assert left.dtype == right.dtype
+    left_na = left.isna()
+    right_na = right.isna()
+    assert_numpy_array_equal(left_na, right_na)
+
+    left_valid = left[~left_na].astype(object)
+    right_valid = right[~right_na].astype(object)
+
+    assert_numpy_array_equal(left_valid, right_valid)
+
+
 # This could be refactored to use the NDFrame.equals method
 def assert_series_equal(left, right, check_dtype=True,
                         check_index_type='equiv',