Not sure where to start with this. The whole point of duck typing is to enable the kind of code reuse you describe, without having to think about virtualising either the library or the calling code explicitly. With duck typing, you don't have to say "I no longer implement IScoobyGang but I do implement IPowerRangers" - remove the method, and it's done.
I understand the rationale behind explicit interfaces (you make a good point when you say "The calling code should check whether a given object implements a particular interface at runtime, and if not then it can handle that appropriately, rather than assuming that my code has a particular method and then crashing mid-function" - this can be done in dynamic languages, but it's a bit more of a faff), but for the kind of code I write, they're overkill. And I have a very low tolerance for writing unnecessary lines of code.
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I understand the rationale behind explicit interfaces (you make a good point when you say "The calling code should check whether a given object implements a particular interface at runtime, and if not then it can handle that appropriately, rather than assuming that my code has a particular method and then crashing mid-function" - this can be done in dynamic languages, but it's a bit more of a faff), but for the kind of code I write, they're overkill. And I have a very low tolerance for writing unnecessary lines of code.