> [...] what makes you implicitly know how the 'IN' behaves in python, and that it won't convert to the keys() method of the dict to get a list with the same behaviour as in a list?

Because it would be the dumb way to do it. You have a lookup procedure to extract a value from a hash. A procedure to check if the key is present in the hash will look exactly the same, barring the value being returned.

Libraries usually work the same way as you would write yourself if you were trained in the task, so it's easy to guess.