Well, I misunderstood your point then.

On your example, you're of course right. I'm also not a fan of checking the actual type in a dynamic language, since it defeats the purpose of it being dynamic. I like assertions that are more useful than that, like:

    def sqrt(x):
        assert x >= 0, "only defined for positive numbers"

        last_guess = x / 2.0
        while True:
            guess = (last_guess + x / last_guess) / 2
            if abs(guess - last_guess) < .000001: 
                return guess
            last_guess = guess

Now clearly this helps, since it aids in readability (this function is defined for positive numbers only) and if you call it with a negative number, it will loop forever.