add tm.assert_cow_warning and use everywhere

pandas/_testing/__init__.py

@@ -88,6 +88,7 @@
     get_obj,
 )
 from pandas._testing.contexts import (
+    assert_cow_warning,
     decompress_file,
     ensure_clean,
     raises_chained_assignment_error,
@@ -1097,6 +1098,7 @@ def shares_memory(left, right) -> bool:
     "assert_series_equal",
     "assert_sp_array_equal",
     "assert_timedelta_array_equal",
+    "assert_cow_warning",
     "at",
     "BOOL_DTYPES",
     "box_expected",

pandas/_testing/contexts.py

@@ -214,3 +214,29 @@ def raises_chained_assignment_error(extra_warnings=(), extra_match=()):
             (ChainedAssignmentError, *extra_warnings),
             match="|".join((match, *extra_match)),
         )
+
+
+def assert_cow_warning(warn=True, match=None, **kwargs):
+    """
+    Assert that a warning is raised in the CoW warning mode.
+
+    Parameters
+    ----------
+    warn : bool, default True
+        By default, check that a warning is raised. Can be turned off by passing False.
+    match : str
+        The warning message to match against, if different from the default.
+    kwargs
+        Passed through to assert_produces_warning
+    """
+    from pandas._testing import assert_produces_warning
+
+    if not warn:
+        from contextlib import nullcontext
+
+        return nullcontext()
+
+    if not match:
+        match = "Setting a value on a view"
+
+    return assert_produces_warning(FutureWarning, match=match, **kwargs)

pandas/tests/copy_view/test_indexing.py

@@ -907,7 +907,7 @@ def test_column_as_series(
         s[0] = 0
     else:
         if warn_copy_on_write:
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_cow_warning():
                 s[0] = 0
         else:
             warn = SettingWithCopyWarning if dtype_backend == "numpy" else None
@@ -939,8 +939,7 @@ def test_column_as_series_set_with_upcast(
 
     s = df["a"]
     if dtype_backend == "nullable":
-        warn = FutureWarning if warn_copy_on_write else None
-        with tm.assert_produces_warning(warn):
+        with tm.assert_cow_warning(warn_copy_on_write):
             with pytest.raises(TypeError, match="Invalid value"):
                 s[0] = "foo"
         expected = Series([1, 2, 3], name="a")

pandas/tests/copy_view/test_methods.py

@@ -1706,8 +1706,7 @@ def test_xs(
     if using_copy_on_write or is_view:
         result.iloc[0] = 0
     elif warn_copy_on_write:
-        warn = FutureWarning if single_block else None
-        with tm.assert_produces_warning(warn):
+        with tm.assert_cow_warning(single_block):
             result.iloc[0] = 0
     else:
         with pd.option_context("chained_assignment", "warn"):

pandas/tests/copy_view/test_setitem.py

@@ -150,9 +150,7 @@ def test_set_column_with_inplace_operator(using_copy_on_write, warn_copy_on_writ
         df["a"] += 1
 
     # when it is not in a chain, then it should produce a warning
-    warn = FutureWarning if warn_copy_on_write else None
-
     df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
     ser = df["a"]
-    with tm.assert_produces_warning(warn):
+    with tm.assert_cow_warning(warn_copy_on_write):
         ser += 1

pandas/tests/frame/indexing/test_setitem.py

@@ -1292,8 +1292,7 @@ def test_setitem_column_update_inplace(
         values = df._mgr.blocks[0].values
 
         if not using_copy_on_write:
-            warn = FutureWarning if warn_copy_on_write else None
-            with tm.assert_produces_warning(warn):
+            with tm.assert_cow_warning(warn_copy_on_write):
                 for label in df.columns:
                     df[label][label] = 1
 

pandas/tests/frame/test_arithmetic.py

@@ -2036,8 +2036,7 @@ def test_inplace_arithmetic_series_update(using_copy_on_write, warn_copy_on_writ
     series = df["A"]
     vals = series._values
 
-    warn = FutureWarning if warn_copy_on_write else None
-    with tm.assert_produces_warning(warn):
+    with tm.assert_cow_warning(warn_copy_on_write):
         series += 1
     if using_copy_on_write:
         assert series._values is not vals

pandas/tests/frame/test_block_internals.py

@@ -348,7 +348,7 @@ def test_stale_cached_series_bug_473(self, using_copy_on_write, warn_copy_on_wri
                 with tm.raises_chained_assignment_error():
                     Y["g"]["c"] = np.nan
             elif warn_copy_on_write:
-                with tm.assert_produces_warning(FutureWarning):
+                with tm.assert_cow_warning():
                     Y["g"]["c"] = np.nan
             else:
                 Y["g"]["c"] = np.nan

pandas/tests/frame/test_constructors.py

@@ -302,8 +302,7 @@ def test_constructor_dtype_nocast_view_dataframe(
             should_be_view.iloc[0, 0] = 99
             assert df.values[0, 0] == 1
         else:
-            warn = FutureWarning if warn_copy_on_write else None
-            with tm.assert_produces_warning(warn):
+            with tm.assert_cow_warning(warn_copy_on_write):
                 should_be_view[0][0] = 99
             assert df.values[0, 0] == 99
 
@@ -313,8 +312,7 @@ def test_constructor_dtype_nocast_view_2d_array(
         df = DataFrame([[1, 2], [3, 4]], dtype="int64")
         if not using_array_manager and not using_copy_on_write:
             should_be_view = DataFrame(df.values, dtype=df[0].dtype)
-            warn = FutureWarning if warn_copy_on_write else None
-            with tm.assert_produces_warning(warn):
+            with tm.assert_cow_warning(warn_copy_on_write):
                 should_be_view[0][0] = 97
             assert df.values[0, 0] == 97
         else:

pandas/tests/indexing/multiindex/test_setitem.py

@@ -279,14 +279,13 @@ def test_series_setitem(
         ymd = multiindex_year_month_day_dataframe_random_data
         s = ymd["A"]
 
-        warn = FutureWarning if warn_copy_on_write else None
-        with tm.assert_produces_warning(warn, match="Setting a value on a view"):
+        with tm.assert_cow_warning(warn_copy_on_write):
             s[2000, 3] = np.nan
         assert isna(s.values[42:65]).all()
         assert notna(s.values[:42]).all()
         assert notna(s.values[65:]).all()
 
-        with tm.assert_produces_warning(warn, match="Setting a value on a view"):
+        with tm.assert_cow_warning(warn_copy_on_write):
             s[2000, 3, 10] = np.nan
         assert isna(s.iloc[49])
 
@@ -541,7 +540,7 @@ def test_frame_setitem_copy_raises(
             df["foo"]["one"] = 2
     elif warn_copy_on_write:
         # TODO(CoW-warn) should warn
-        with tm.assert_produces_warning(None):
+        with tm.assert_cow_warning(False):
             df["foo"]["one"] = 2
     else:
         msg = "A value is trying to be set on a copy of a slice from a DataFrame"
@@ -560,7 +559,7 @@ def test_frame_setitem_copy_no_write(
             df["foo"]["one"] = 2
     elif warn_copy_on_write:
         # TODO(CoW-warn) should warn
-        with tm.assert_produces_warning(None):
+        with tm.assert_cow_warning(False):
             df["foo"]["one"] = 2
     else:
         msg = "A value is trying to be set on a copy of a slice from a DataFrame"

pandas/tests/indexing/test_chaining_and_caching.py

@@ -218,10 +218,9 @@ def test_detect_chained_assignment(self, using_copy_on_write, warn_copy_on_write
                     df["A"][1] = -6
                 tm.assert_frame_equal(df, df_original)
             else:
-                warn = FutureWarning if warn_copy_on_write else None
-                with tm.assert_produces_warning(warn):
+                with tm.assert_cow_warning(warn_copy_on_write):
                     df["A"][0] = -5
-                with tm.assert_produces_warning(warn):
+                with tm.assert_cow_warning(warn_copy_on_write):
                     df["A"][1] = -6
                 tm.assert_frame_equal(df, expected)
 
@@ -246,9 +245,9 @@ def test_detect_chained_assignment_raises(
                 df["A"][1] = -6
             tm.assert_frame_equal(df, df_original)
         elif warn_copy_on_write:
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_cow_warning():
                 df["A"][0] = -5
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_cow_warning():
                 df["A"][1] = np.nan
         elif not using_array_manager:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -283,8 +282,8 @@ def test_detect_chained_assignment_fails(
             with tm.raises_chained_assignment_error():
                 df.loc[0]["A"] = -5
         elif warn_copy_on_write:
-            # TODO(CoW) should warn
-            with tm.assert_produces_warning(None):
+            # TODO(CoW-warn) should warn
+            with tm.assert_cow_warning(False):
                 df.loc[0]["A"] = -5
         else:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -308,8 +307,8 @@ def test_detect_chained_assignment_doc_example(
             with tm.raises_chained_assignment_error():
                 df[indexer]["c"] = 42
         elif warn_copy_on_write:
-            # TODO(CoW) should warn
-            with tm.assert_produces_warning(None):
+            # TODO(CoW-warn) should warn
+            with tm.assert_cow_warning(False):
                 df[indexer]["c"] = 42
         else:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -333,7 +332,7 @@ def test_detect_chained_assignment_object_dtype(
             tm.assert_frame_equal(df, df_original)
         elif warn_copy_on_write:
             # TODO(CoW-warn) should give different message
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_cow_warning():
                 df["A"][0] = 111
             tm.assert_frame_equal(df, expected)
         elif not using_array_manager:
@@ -463,7 +462,7 @@ def test_detect_chained_assignment_undefined_column(
             tm.assert_frame_equal(df, df_original)
         elif warn_copy_on_write:
             # TODO(CoW-warn) should warn
-            with tm.assert_produces_warning(None):
+            with tm.assert_cow_warning(False):
                 df.iloc[0:5]["group"] = "a"
         else:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -494,10 +493,10 @@ def test_detect_chained_assignment_changing_dtype(
             tm.assert_frame_equal(df, df_original)
         elif warn_copy_on_write:
             # TODO(CoW-warn) should warn
-            with tm.assert_produces_warning(None):
+            with tm.assert_cow_warning(False):
                 df.loc[2]["D"] = "foo"
             # TODO(CoW-warn) should give different message
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_cow_warning():
                 df["C"][2] = "foo"
         else:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -529,7 +528,7 @@ def test_setting_with_copy_bug(self, using_copy_on_write, warn_copy_on_write):
             tm.assert_frame_equal(df, df_original)
         elif warn_copy_on_write:
             # TODO(CoW-warn) should warn
-            with tm.assert_produces_warning(None):
+            with tm.assert_cow_warning(False):
                 df[["c"]][mask] = df[["b"]][mask]
         else:
             with pytest.raises(SettingWithCopyError, match=msg):
@@ -554,7 +553,7 @@ def test_detect_chained_assignment_warnings_errors(
             return
         elif warn_copy_on_write:
             # TODO(CoW-warn) should warn
-            with tm.assert_produces_warning(None):
+            with tm.assert_cow_warning(False):
                 df.loc[0]["A"] = 111
             return
 

pandas/tests/series/accessors/test_dt_accessor.py

@@ -294,9 +294,9 @@ def test_dt_accessor_not_writeable(self, using_copy_on_write, warn_copy_on_write
                 with tm.raises_chained_assignment_error():
                     ser.dt.hour[0] = 5
             elif warn_copy_on_write:
-                # TODO should warn
-                # with tm.assert_produces_warning(FutureWarning):
-                ser.dt.hour[0] = 5
+                # TODO(CoW-warn) should warn
+                with tm.assert_cow_warning(False):
+                    ser.dt.hour[0] = 5
             else:
                 with pytest.raises(SettingWithCopyError, match=msg):
                     ser.dt.hour[0] = 5

pandas/tests/series/methods/test_rename.py

@@ -167,8 +167,7 @@ def test_rename_copy_false(self, using_copy_on_write, warn_copy_on_write):
         ser = Series(["foo", "bar"])
         ser_orig = ser.copy()
         shallow_copy = ser.rename({1: 9}, copy=False)
-        warn = FutureWarning if warn_copy_on_write else None
-        with tm.assert_produces_warning(warn):
+        with tm.assert_cow_warning(warn_copy_on_write):
             ser[0] = "foobar"
         if using_copy_on_write:
             assert ser_orig[0] == shallow_copy[0]