Improve performance of deepcopy by setting keep alive key in the memo

Feature or enhancement

Proposal:

In the deepcopy implementation an exception is generated when the memo dict does not contain a key id(memo). We can avoid generating an exception by adding this key when the memo is generated. As a consequence the _keep_alive method is just a single line of code and we can inline this.

There is a behavior change: a call deepcopy(x, {}) works with current main, but will raise an exception (since the key id(memo)) has not been set. This might be acceptable as the documentation states "The memo dictionary should be treated as an opaque object." (see https://docs.python.org/3/library/copy.html). And in the code itself the memo argument is not documented at all https://github.com/python/cpython/blob/9aea9c100fbbaa1437f595da65417204872106a9/Lib/copy.py#L3-L9

There are some unit tests involving the memo, but those seem to test the behavior of the deepcopy and not of the memo argument.

If the behavior change is not acceptable because memo is part of the public interface, should we then instead document the structure of the argument memo` as being an implementation detail?

The argument _nil as a sentinel to the deepcopy is redundant, as the memo can only contain integers so we can use None instead.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-123746

Start by reading Lib/copy.py and the existing unit tests that exercise deepcopy with a memo. Review linked PR gh-123746 and resolve whether the memo behavior is a public-interface change; done means the chosen implementation, tests, and any needed documentation consistently define the behavior.