PDEP-7: https://pandas.pydata.org/pdeps/0007-copy-on-write.html

An initial implementation was merged in #46958 (with the proposal described in more detail in https://docs.google.com/document/d/1ZCQ9mx3LBMy-nhwRl33_jgcvWo9IWdEfxDNQ2thyTb0/edit / discussed in #36195).

In #36195 (comment) I mentioned some next steps that are still needed; moving this to a new issue.

Implementation

Complete the API surface:

• Use the lazy copy (with CoW) mechanism in more methods where it can be used. Overview issue at ENH / CoW: Use the "lazy copy" (with Copy-on-Write) optimization in more methods where appropriate #49473
• What to do with the existing copy keyword?
  • API: CoW and explicit copy keyword in DataFrame/Series methods #50535 -> CoW: Ignore copy=True when copy_on_write is enabled #51464
  • DEPR (PDEP-7/CoW): deprecate and remove copy keyword (except in constructors) #56022
• Implement the new behaviour for constructors (eg constructing a DataFrame/Series from an existing DataFrame/Series should follow the same rules as indexing -> behaves as a copy through CoW). Although, what about creating from a numpy array?
  • API / CoW: constructing Series from Series creates lazy copy (with default copy=False) #49524
  • API / CoW: constructing DataFrame from DataFrame/BlockManager creates lazy copy #51239
  • API / CoW: DataFrame(<dict of Series>, copy=False) constructor now gives lazy copy #50777
  • API / CoW: copy/view behaviour when constructing DataFrame/Series from a numpy array #50776
    • API / CoW: Copy NumPy arrays by default in DataFrame constructor #51731
    • API / CoW: Copy arrays by default in Series constructor #52022
  • Constructing DataFrame/Series from Index object (also keep track of references): API / CoW: Respect CoW for DataFrame(Index) #52276 and CoW: Avoid copying Index in Series constructor #52008 and CoW: Add lazy copy mechanism to DataFrame constructor for dict of Index #52947
• Explore / update the APIs that return numpy arrays (.values, to_numpy()). Potential idea is to make the returned array read-only by default.
  • API / CoW: return read-only numpy arrays in .values/to_numpy() #51082
  • CoW: Return read-only array in Index.values #53704
  • We need to do the same for EAs? (now only for numpy arrays) -> CoW: add readonly flag to ExtensionArrays, return read-only EA/ndarray in .array/EA.to_numpy() #61925
• Warning / error for chained setitem that no longer works -> API / CoW: detect and raise error for chained assignment under Copy-on-Write #49467
  • BUG: ChainedAssignmentError for CoW not working when setitem is called from cython #51315
  • BUG: ChainedAssignmentError for CoW not working for chained inplace methods when passing *args or **kwargs #56456
  • Add the same warning/error for inplace methods that are called in a chained context (eg df['a'].fillna(.., inplace=True)
    • CoW: Add warning for chained assignment with fillna #53779
    • CoW: Add ChainedAssignmentError for update #54024
    • CoW: Add ChainedAssignmentError for replace with inplace=True #54023

Improve the performance

• Optimize setitem operations to prevent copies of whole blocks (eg splitting the block could help keeping a view for all other columns, and we only take a copy for the columns that are modified) where splitting the block could keep a view for all other columns, and
  • ENH: Avoid copying unnecessary columns in setitem by splitting blocks for CoW #51031
• Check overall performance impact (eg run asv with / without CoW enabled by default and see the difference)

Provide upgrade path:

• Add a warning mode that gives deprecation warnings for all cases where the current behaviour would change (initially also behind an option): CoW warning mode for cases that will change behaviour #56019
  • We can also update the message of the existing SettingWithCopyWarnings to point users towards enabling CoW as a way to get rid of the warnings
  • Add a general FutureWarning "on first use that would change" that is only raised a single time

Documentation / feedback

Aside from finalizing the implementation, we also need to start documenting this, and it will be super useful to have people give this a try, run their code or test suites with it, etc, so we can iron out bugs / missing warnings / or discover unexpected consequences that need to be addressed/discussed.

• Document this new feature (how it works, how you can test it)
• We can still add a note to the 1.5 whatsnew linking to those docs
• Write a set of blogposts on the topic
• Gather feedback from users / downstream packages
• Update existing documentation:
  • DOC: Add note about CoW change to copy keyword docs #56162
  • [ ]
• Write an upgrade guide

Some remaining aspects of the API to figure out:

• What to do with the Series.view() method -> is deprecated
• Let head()/tail() return eager copies? (to avoid using those methods for exploration trigger CoW) -> API/CoW: Return copies for head and tail #54011