BUG: Fix replacing in string series with NA (pandas-dev#32621)

Author: chrispe

pandas/core/internals/managers.py

@@ -28,7 +28,7 @@
 from pandas.core.dtypes.concat import concat_compat
 from pandas.core.dtypes.dtypes import ExtensionDtype
 from pandas.core.dtypes.generic import ABCDataFrame, ABCSeries
-from pandas.core.dtypes.missing import isna, na_value_for_dtype
+from pandas.core.dtypes.missing import isna
 
 import pandas.core.algorithms as algos
 from pandas.core.arrays.sparse import SparseDtype
@@ -1948,22 +1948,28 @@ def _compare_or_regex_search(a, b, regex=False):
     is_a_array = isinstance(a, np.ndarray)
     is_b_array = isinstance(b, np.ndarray)
 
-    # GH#32621 replace all pd.NAs to avoid failure of element-wise comparison
-    mask = isna(a) | isna(b)
-    if is_a_array:
-        a = np.where(mask, na_value_for_dtype(a.dtype, compat=False), a)
-    if is_b_array:
-        b = np.where(mask, na_value_for_dtype(b.dtype, compat=False), b)
-
     if is_datetimelike_v_numeric(a, b) or is_numeric_v_string_like(a, b):
         # GH#29553 avoid deprecation warnings from numpy
         result = False
     else:
+        # GH#32621 use mask to avoid comparing to NAs
+        if is_a_array and not is_b_array:
+            mask = np.reshape(~(isna(a)), a.shape)
+        elif is_b_array and not is_a_array:
+            mask = np.reshape(~(isna(b)), b.shape)
+        elif is_a_array and is_b_array:
+            mask = ~(isna(a) | isna(b))
+
+        if is_a_array:
+            a = a[mask]
+        if is_b_array:
+            b = b[mask]
         result = op(a)
+
         if isinstance(result, np.ndarray):
-            result[mask] = na_value_for_dtype(result.dtype, compat=False)
-        elif isna(result):
-            result = na_value_for_dtype(np.bool, compat=False)
+            tmp = np.zeros(mask.shape, dtype=np.bool)
+            tmp[mask] = result
+            result = tmp
 
     if is_scalar(result) and (is_a_array or is_b_array):
         type_names = [type(a).__name__, type(b).__name__]