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Python Python Collections (2016, retired 2019) Sets Set Math

Posted by Niki Esfandiari
Niki Esfandiari
4,160 Points

I don't understand why this code won't work for the 'covers' question in the sets course

def covers(set1): ans = [] for course, topics in COURSES.items(): if len(set1.union(topics)) > 0: ans.append(course) return ans

sets.py 
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"}
}

def covers(set1):
    ans = []
    for course, topics in COURSES.items():
        if len(set1.union(topics)) > 0:
            ans.append(course)
    return ans
Ewerton Luna
.a{fill-rule:evenodd;}techdegree seal-36
Ewerton Luna
Full Stack JavaScript Techdegree Graduate 24,031 Points

Your code won't work because it will always return a list with all the courses in it.

Why? Take a look at this line of code: 

if len(set1.union(topics)) > 0: 
    ans.append(course)

This if statement will always return True. This topics variable will always have len > 1, since the key values in the COURSES dict are not empty. Even if you pass set1 as an empty set, len(set1.union(topics)) will still be > 1 for the reason I just mentioned. So your list nammed ans will be appended with all the courses.

I made a few changes on your code:

    def covers2(set1):
        ans = []
        for course, topics in COURSES.items():
            for topic in set1:
                if topic in topics:
                    ans.append(course)
        ans = list(set(ans))
        return ans

Remember to coerce ans into a set, otherwise the code will return a list with repeated items in it.

I just passed this challenge and my code was the following:

def covers(set_of_topics):
    result = set({})
    for topic in set_of_topics:
        for course in COURSES:
            if topic in COURSES[course]:
                result.add(course)
    return list(result)

Just as a side note, try using a desktop IDE like PyCharm. Testing your code there will help you a lot.