> Sequence[SupportsFloat] | Mapping[int,SupportsFloat]

This is really just the same mistake as the original expanding union, but with overly narrow abstract types instead of overly narrow concrete types. If it relies on “we can use indexing with an int and get out something whose type we don’t care about”, then its a Protocol with the following method:

  def __getitem__(self, i: int, /) -> Any: ...

More generally, even if there is a specific output type when indexing, or the output type of indexing can vary but in a way that impacts the output or other input types of the function, it is a protocol with a type parameter T and this method:

  def __getitem__(self, i: int, /) -> T: ...

It doesn’t need to be union of all possible concrete and/or abstract types that happen to satisfy that protocol, because it can be expressed succinctly and accurately in a single Protocol.