TST/CoW: expand test for chained inplace methods (#56402)

* TST/CoW: expand test for chained inplace methods

* fix test for older python

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: jorisvandenbossche

pandas/tests/copy_view/test_chained_assignment_deprecation.py

@@ -1,6 +1,7 @@
 import numpy as np
 import pytest
 
+from pandas.compat import PY311
 from pandas.errors import (
     ChainedAssignmentError,
     SettingWithCopyWarning,
@@ -42,7 +43,9 @@ def test_methods_iloc_warn(using_copy_on_write):
         ("ffill", ()),
     ],
 )
-def test_methods_iloc_getitem_item_cache(func, args, using_copy_on_write):
+def test_methods_iloc_getitem_item_cache(
+    func, args, using_copy_on_write, warn_copy_on_write
+):
     # ensure we don't incorrectly raise chained assignment warning because
     # of the item cache / iloc not setting the item cache
     df_orig = DataFrame({"a": [1, 2, 3], "b": 1})
@@ -66,14 +69,74 @@ def test_methods_iloc_getitem_item_cache(func, args, using_copy_on_write):
     ser = df["a"]
     getattr(ser, func)(*args, inplace=True)
 
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    # TODO(CoW-warn) because of the usage of *args, this doesn't warn on Py3.11+
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error(not PY311):
+            getattr(df["a"], func)(*args, inplace=True)
+    else:
+        with tm.assert_cow_warning(not PY311, match="A value"):
+            getattr(df["a"], func)(*args, inplace=True)
+
+    df = df_orig.copy()
+    ser = df["a"]  # populate the item_cache and keep ref
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error(not PY311):
+            getattr(df["a"], func)(*args, inplace=True)
+    else:
+        # ideally also warns on the default mode, but the ser' _cacher
+        # messes up the refcount + even in warning mode this doesn't trigger
+        # the warning of Py3.1+ (see above)
+        with tm.assert_cow_warning(warn_copy_on_write and not PY311, match="A value"):
+            getattr(df["a"], func)(*args, inplace=True)
+
+
+def test_methods_iloc_getitem_item_cache_fillna(
+    using_copy_on_write, warn_copy_on_write
+):
+    # ensure we don't incorrectly raise chained assignment warning because
+    # of the item cache / iloc not setting the item cache
+    df_orig = DataFrame({"a": [1, 2, 3], "b": 1})
+
+    df = df_orig.copy()
+    ser = df.iloc[:, 0]
+    ser.fillna(1, inplace=True)
+
+    # parent that holds item_cache is dead, so don't increase ref count
+    df = df_orig.copy()
+    ser = df.copy()["a"]
+    ser.fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df.iloc[:, 0]  # iloc creates a new object
+    ser.fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    df["a"]  # populate the item_cache
+    ser = df["a"]
+    ser.fillna(1, inplace=True)
+
     df = df_orig.copy()
     df["a"]  # populate the item_cache
     if using_copy_on_write:
         with tm.raises_chained_assignment_error():
-            df["a"].fillna(0, inplace=True)
+            df["a"].fillna(1, inplace=True)
     else:
         with tm.assert_cow_warning(match="A value"):
-            df["a"].fillna(0, inplace=True)
+            df["a"].fillna(1, inplace=True)
+
+    df = df_orig.copy()
+    ser = df["a"]  # populate the item_cache and keep ref
+    if using_copy_on_write:
+        with tm.raises_chained_assignment_error():
+            df["a"].fillna(1, inplace=True)
+    else:
+        # TODO(CoW-warn) ideally also warns on the default mode, but the ser' _cacher
+        # messes up the refcount
+        with tm.assert_cow_warning(warn_copy_on_write, match="A value"):
+            df["a"].fillna(1, inplace=True)
 
 
 # TODO(CoW-warn) expand the cases