BUG: Keep original values when taking with a new fill value (#55929)

* Keep original values when taking with a new fill value

* Switch one-letter name

* Fix whatsnew

Author: rob-sil

doc/source/whatsnew/v2.2.0.rst

@@ -467,7 +467,7 @@ Reshaping
 
 Sparse
 ^^^^^^
--
+- Bug in :meth:`SparseArray.take` when using a different fill value than the array's fill value (:issue:`55181`)
 -
 
 ExtensionArray

pandas/core/arrays/sparse/array.py

@@ -1086,9 +1086,10 @@ def _take_with_fill(self, indices, fill_value=None) -> np.ndarray:
             )
 
         elif self.sp_index.npoints == 0:
-            # Avoid taking from the empty self.sp_values
+            # Use the old fill_value unless we took for an index of -1
             _dtype = np.result_type(self.dtype.subtype, type(fill_value))
             taken = np.full(sp_indexer.shape, fill_value=fill_value, dtype=_dtype)
+            taken[old_fill_indices] = self.fill_value
         else:
             taken = self.sp_values.take(sp_indexer)
 

pandas/tests/arrays/sparse/test_indexing.py

@@ -166,9 +166,16 @@ def test_take(self, arr_data, arr):
         tm.assert_sp_array_equal(arr.take([0, 1, 2]), exp)
 
     def test_take_all_empty(self):
-        a = pd.array([0, 0], dtype=SparseDtype("int64"))
-        result = a.take([0, 1], allow_fill=True, fill_value=np.nan)
-        tm.assert_sp_array_equal(a, result)
+        sparse = pd.array([0, 0], dtype=SparseDtype("int64"))
+        result = sparse.take([0, 1], allow_fill=True, fill_value=np.nan)
+        tm.assert_sp_array_equal(sparse, result)
+
+    def test_take_different_fill_value(self):
+        # Take with a different fill value shouldn't overwrite the original
+        sparse = pd.array([0.0], dtype=SparseDtype("float64", fill_value=0.0))
+        result = sparse.take([0, -1], allow_fill=True, fill_value=np.nan)
+        expected = pd.array([0, np.nan], dtype=sparse.dtype)
+        tm.assert_sp_array_equal(expected, result)
 
     def test_take_fill_value(self):
         data = np.array([1, np.nan, 0, 3, 0])