CoW warning mode for cases that will change behaviour

[jorisvandenbossche]

We are adding a new "warning" mode for the Copy-on-Write option (#48998):

pd.options.mode.copy_on_write = "warn"

which will raise a warning in every case that you are doing a setitem operation that will change behaviour when Copy-on-Write is enabled (i.e. when you are setting on an object that shares a view with another object, and with current pandas would also update that other object)

This is a tracking issue for the subtasks to complete this:

• Initial PR + tackling Series setitem (mgr.setitem_inplace): CoW: add warning mode for cases that will change behaviour #55428
• Single block setitem (mgr.setitem): CoW warning mode: add warning for single block setitem #55838
• Setting values into single column (mgr.column_setitem): CoW warning mode: setting values into single column of DataFrame #56020
• mgr.iset? (mostly used from mgr.column_setitem which already raises the warning?)
• mgr._iset_single? (this seems only covered by test_internals.py::TestBlockManager::test_get_numeric_data/test_get_bool_data, and so this is probably not possible to get to through public APIs)
• Chained setitem: CoW warning mode: warn in case of chained assignment #55522
• Chained inplace methods (fillna etc)
  • CoW: Add warning for replace with inplace #56060
  • CoW: Add warning for fillna with inplace #56064
  • CoW: Add warning for interpolate with inplace #56066
  • CoW: Add warning for mask, where and clip with inplace #56067
• CoW: Warn for cases that go through putmask #56168
• False positives with inplace operators (eg ser[0:2] += 1) -> CoW: Remove false positive warnings for inplace operators #56242
• Update the Stata reader code to not trigger warnings (check general ignore in test_stata.py): CoW: Avoid warnings in stata code #56392

In addition, there are also a bunch of remaining # TODO(CoW-warn) to resolve.

[mroeschke]

Since CoW is enabled on main I think we can close this issue