Add copyreg.copy() and copyreg.deepcopy()

[serhiy-storchaka]

Feature or enhancement

Proposal:

Add two registration functions to copyreg, following the copyreg.pickle() precedent:

• copyreg.copy(type, function) — registers the shallow copy function for objects of type type; called as function(obj), like __copy__.
• copyreg.deepcopy(type, function) — registers the deep copy function; called as function(obj, memo), like __deepcopy__.

copy.copy() and copy.deepcopy() use the registered functions in preference to the special methods and the pickle interfaces. Registration is by exact type and does not affect pickling. This allows customizing copying of instances of types that cannot be modified — for example, treating an external library's immutable objects as atomic:

copyreg.deepcopy(thirdparty.Handle, lambda obj, memo: obj)

Today the only way to customize copying of a foreign type is copyreg.pickle(), which forces copy and pickle semantics to be the same. The private _copy_dispatch and _deepcopy_dispatch tables have been monkey-patched in the wild for lack of a public API — even by our own test_descr. The new registries replace this non-public way; _deepcopy_dispatch is now removed, as _copy_dispatch already was in gh-128118.

This was requested in gh-141737. Implementation is ready; PR to follow.

Has this already been discussed elsewhere?

Companion of PEP 837, whose Rationale notes that registries for other protocols will be proposed separately: https://discuss.python.org/t/pep-837-extensible-json-serialization/108124

Links to previous discussion of this feature:

• [deepcopy] Support passing some additional classes to be considered for atomic deepcopy #141737 — the direct feature request
• Improve performance of copy.copy #128118 — removal of the historical _copy_dispatch table

Linked PRs

• gh-153612: Add copyreg.copy() and copyreg.deepcopy() #153613