gh-104050: Argument Clinic: Annotate the Block class

[AlexWaygood]

I tried long and hard to see if there was a sane way of annotating the output field of the Block class. In the end I concluded that, from a typing perspective, the current code is somewhat insane, so there's no sane way of doing it without substantially refactoring the code first 🙃 For now, I propose that we annotate it with Any and leave it for another day.

The issue is that output usually has type str | None, as stated by the docstring here:

cpython/Tools/clinic/clinic.py

Lines 1659 to 1660 in 363f4f9

	output is either str or None. If str, it's the output
	from this block, with embedded '\n' characters.

But whenever self.block is referenced from inside the DSLParser class, it's of type list[str] (I'm pretty confident self.block is an instance of Block):

cpython/Tools/clinic/clinic.py

Lines 4471 to 4476 in 363f4f9

	def directive_dump(self, name: str) -> None:
	self.block.output.append(self.clinic.get_destination(name).dump())
	def directive_printout(self, *args: str) -> None:
	self.block.output.append(' '.join(args))
	self.block.output.append('\n')

This is because DSLParser.parse() sets block.output to a list at the beginning of the function, and then sets it back to a string at the end of the function:

cpython/Tools/clinic/clinic.py

Line 4502 in 363f4f9

block.output = []

cpython/Tools/clinic/clinic.py

Lines 4515 to 4518 in 363f4f9

	if self.preserve_output:
	if block.output:
	fail("'preserve' only works for blocks that don't produce any output!")
	block.output = self.saved_output

Needless to say, this makes mypy furious if you give Block.output a type that isn't Any 🙃

• Issue: Add type annotations to clinic.py #104050

[erlend-aasland]

Good digging, there! Wow, yeah, we need to refactor this piece of code :)