Practically speaking, one issue I have is how easy it is to write isinstance or issubclass checks. It has historically been much more difficult to write and maintain a check that something looks like a duck.

`if hasattr(foo, 'close') and hasattr(foo, 'seek') and hasattr(foo, 'read'):`

Just does not roll off the figurative tongue and that is a relatively simple example of what is required for a duck check.

To prevent isinstance use when a duck check would be better,

I'm going to chime in briefly then return to lurking on this topic, trying to figure out all the changes to typing while I wasn't paying attention. Back in ancient times I recall "look before you leap" as the description of either of the above styles of checks, no matter which was easier to type. At the time, I thought the general recommendation was to document what attributes you expected objects to provide and just make the relevant unguarded references. I no longer recall what the tongue-in-cheek description of that style was (just "leap"?) Is that more simple usage more akin to classic "duck typing" than always guarding accesses? I assume that will still have a place in the pantheon of Python type variants.

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