Just the visual clutter of adding type annotations can make the code flow less immediately clear and then due to broken windows syndrome people naturally care less and less about visual clarity.
I disagree strongly, based on 20 years of using Python without annotations and ~5 years of seeing people ask questions about how to do advanced things with types. And based on reading Python code, and comparing that to how I feel when reading code in any manifest-typed language.
>Reading Python functions in isolation, you might not even know what data/structure you’re getting as input
I'm concerned with what capabilities the input offers, not the name given to one particular implementation of that set of capabilities. If I have to think about it in any more detail than "`ducks` is an iterable of Ducklike" (n.b.: a code definition for an ABC need not actually exist; it would be dead code that just complicates method resolution) I'm trying to do too much in that function. If I have to care about whether the iterable is a list or a string (given that length-1 strings satisfy the ABC), I'm either trying to do the wrong thing or using the wrong language.
> if there’s something that muddles up immediate clarity it’s ambiguity about what data code is operating on.
There is no ambiguity. There is just disregard for things that don't actually matter, and designing to make sure that they indeed don't matter.
You can specify exactly that and no more, using the type system:
def foo(ducks: Iterable[Ducklike]) -> None:
...
IMO this is the source of much of the demand for type hints in Python. People don't want to write idiomatic Python, they want to write Java - but they're stuck using Python because of library availability or an existing Python codebase.
So, they write Java-style code in Python. Most of the time this means heavy use of type hints and an overuse of class hierarchies (e.g. introducing abstract classes just to satisfy the type checker) - which in my experience leads to code that's twice as long as it should be. But recently I heard more extreme advice - someone recommended "write every function as a member of a class" and "put every class in its own file".
Type checking on the other hand makes duck typing awesome. All the flexibility, none of the surprises.
If you passed a string expecting it to be treated as an atomic value rather than as a sequence (i.e. you made a mistake and want a type checker to catch it for you), there are many other things you can do to avoid creating that expectation in the first place.