Frequently, in Python, code which checks for types, rather than checking for features, ends up being excessively restrictive and insufficiently general.
That's true, but checking for the exact features that are going to be needed by your code is usually quite complicated and painful to maintain. At the beginning, you need only a sequence. In the end, you need a mutable sequence that supports slicing, so you could go by requiring a list as well.
Enforcing types also brings the benefit that the program is more deterministic. In my experience, there is a lot more benefits to have an object whose type is clearly identified than to have function that accepts generic objects.
I would go as far as saying that variables should have immutable types. It is more restricting that what python provides currently, but leads to clearer programming practice: the intent of the developer shows up in the type he uses.
Of course, with languages such as OCaml, you get both of it: liberal typing with enforced consistency.
While Python's dynamic typing liberty is enjoyable, I think it harms when you start to work on big projects. It prevents you to put many safety assumptions which might bite you back.
regards,
Philippe -- http://mail.python.org/mailman/listinfo/python-list
