[dataclass_transform] support implicit default for "init" parameter in field specifiers #15010
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(Basic functionality was implemented by @wesleywright in #14870. I added overload resolution.)
This note from PEP 681 was missed in the initial implementation of field specifiers:
This commit adds support for reading a default from the declared type of the
init
parameter if possible. Otherwise, it continues to use the typical default ofTrue
.The implementation was non-trivial, since regular overload resolution can't be used in the dataclass plugin, which is applied before type checking. As a workaround, I added a simple overload resolution helper that should be enough to support typical use cases. It doesn't do full overload resolution using types, but it knows about
None
,Literal[True]
andLiteral[False]
and a few other things.