Workspaces vs Visual Studio Python Extension

I've been using Treehouse workspaces and also Visual Studio Python Extension as a way to understand how Python works out in the 'real world'. This set exercise can be answered in this way:

def covers(x): courses_list = [] for course in COURSES: if x.intersection(COURSES[course]): courses_list.append(course) return courses_list

This works in workspaces, but doesn't work in VS Python. I get an AttributeError: 'str' object has no attribute 'intersection'.

I can get around this error by changing this line: if x.intersection(COURSES[course]): to this - if set(x).intersection(COURSES[course]):

My thinking here was that the error message was telling me that it thinks 'x' is a string. If I wrap it in set() then it will change it's type.

From my solution above, can you tell me how Python knows 'x' is a 'set' type in workspaces? It's not really defined anywhere. I'm mostly a C# guy so this whole thing about not declaring data types has been really throwing me off. Python seems to be the 'Wild West' to me! Anything goes!

COURSES = {
    "Python Basics": {"Python", "functions", "variables",
                      "booleans", "integers", "floats",
                      "arrays", "strings", "exceptions",
                      "conditions", "input", "loops"},
    "Java Basics": {"Java", "strings", "variables",
                    "input", "exceptions", "integers",
                    "booleans", "loops"},
    "PHP Basics": {"PHP", "variables", "conditions",
                   "integers", "floats", "strings",
                   "booleans", "HTML"},
    "Ruby Basics": {"Ruby", "strings", "floats",
                    "integers", "conditions",
                    "functions", "input"}
}