ENH: Add test for asfreq CoW when doing noop (pandas-dev#50916)

Co-authored-by: Joris Van den Bossche <[email protected]>

Author: 2 people

pandas/tests/copy_view/test_methods.py

@@ -1117,6 +1117,26 @@ def test_putmask(using_copy_on_write):
         assert view.iloc[0, 0] == 5
 
 
+def test_asfreq_noop(using_copy_on_write):
+    df = DataFrame(
+        {"a": [0.0, None, 2.0, 3.0]},
+        index=date_range("1/1/2000", periods=4, freq="T"),
+    )
+    df_orig = df.copy()
+    df2 = df.asfreq(freq="T")
+
+    if using_copy_on_write:
+        assert np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    else:
+        assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+
+    # mutating df2 triggers a copy-on-write for that column / block
+    df2.iloc[0, 0] = 0
+
+    assert not np.shares_memory(get_array(df2, "a"), get_array(df, "a"))
+    tm.assert_frame_equal(df, df_orig)
+
+
 def test_isetitem(using_copy_on_write):
     df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6], "c": [7, 8, 9]})
     df_orig = df.copy()