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Description
Documentation
The documentation for the email.headerregistry module say that the parse() method of custom header classes must ensure that kwds contains the keys decoded and defects:
However, there is a third key required, parse_tree, as mentioned in the source code:
cpython/Lib/email/headerregistry.py
Line 165 in 8edfa0b
The documentation should mention this required key as well. Ideally, it would also explain what its content should be.

