Wrangling the typing PEPs

This is because Python doesn't have a standard "into" method. Instead, the canonical way to convert type X into type Y is to call Y's constructor on an instance of X (just like in C++, except that Python never implicitly calls a constructor when the types don't match - so it's like C++ if all single-argument constructors were explicit). But in Python, the values which are acceptable to Y's constructor are annotated on Y's constructor (or more commonly, on its __init__ method, which is technically not a constructor, but whatever), not on X, so there's simply no obvious way to statically annotate "anything that Y can accept as a constructor argument." Perhaps there should be - but ideally, we would generalize this to support all functions, not just __init__, so that you could instead write "anything that is acceptable to the foo function as an argument," with Y.__init__ being one possible value of "the foo function."

If you are writing Y yourself, you can fix this by using whatever annotation you used on Y's constructor, and you can even set up a convenient TypeAlias for it if desired.