BUG: Count creates result with read_only array (#51943)

Author: phofl

pandas/core/frame.py

@@ -10870,7 +10870,7 @@ def count(self, axis: Axis = 0, numeric_only: bool = False):
             else:
                 # GH13407
                 series_counts = notna(frame).sum(axis=axis)
-                counts = series_counts.values
+                counts = series_counts._values
                 result = self._constructor_sliced(
                     counts, index=frame._get_agg_axis(axis)
                 )

pandas/tests/copy_view/test_methods.py

@@ -1674,6 +1674,14 @@ def test_transpose_ea_single_column(using_copy_on_write):
     assert not np.shares_memory(get_array(df, "a"), get_array(result, 0))
 
 
+def test_count_read_only_array():
+    df = DataFrame({"a": [1, 2], "b": 3})
+    result = df.count()
+    result.iloc[0] = 100
+    expected = Series([100, 2], index=["a", "b"])
+    tm.assert_series_equal(result, expected)
+
+
 def test_series_view(using_copy_on_write):
     ser = Series([1, 2, 3])
     ser_orig = ser.copy()