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

Jason Smith
Jason Smith
8,668 Points
Posted by Jason Smith
Jason Smith
8,668 Points

it says i didn't get the right output. what am i doing wrong?

i don't really see why this wouldn't work

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 (topic):
    return list(topic.intersection(COURSES))

1 Answer

Alex Koumparos
seal-mask
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points
Alex Koumparos
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Jason,

Let's run your code in the Python interpreter and see what it tells us:

>>> covers({"Python"})
[]

So we're getting an empty list, but the question expects:

For example, covers({"Python"}) would return ["Python Basics"].

Ignoring the type conversion from set to list, your function only has one method call, so it's a pretty safe bet that the issue is in there somewhere.

Let's take a quick look at the official docs for intersection:

intersection(*others)
set & other & ...
Return a new set with elements common to the set and all others.
... Note, the non-operator versions of union(), intersection(), difference(), and symmetric_difference(), issubset(), and issuperset() methods will accept any iterable as an argument.

So, this tells us that the argument to intersection needs to be an iterable, but it can be any iterable.

We know that we're providing an iterable, otherwise our code would do this in the interpreter:

>>> {"Python"}.intersection(3)   # an int is not an iterable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

Since we get an empty list but not a TypeError, we can deduce that we are passing in an iterable, but not the iterable we expect.

What about if we do this:

>>> {"Python"}.intersection({"Python", "functions", "variables", "booleans", "integers", "floats", "arrays", "strings", "exceptions", "conditions", "input", "loops"})
{'Python'}

That looks pretty promising. But if we do this:

>>> {"Python"}.intersection({"Python Basics": {"Python", "functions", "variables", "booleans", "integers", "floats", "arrays", "strings", "exceptions", "conditions", "input", "loops"}})
set()

We get an empty set. What we are seeing is that intersection will only look at each element of the iterable you pass in and compare that to your set. Since the elements of promising version were the topics in the set, it could match "Python" to "Python". But in the dictionary, the elements are not topics, they are dictionary elements, i.e., a key-value pair. In the specific case above, the element is {"Python Basics": {"Python", "functions", etc}} (Strictly, when you pass in the dictionary, intersection() treats the key as the element for comparing to your set, so in this case, just "Python Basics").

Hopefully, this clears up where you are going off-track and gives you some hints for how to proceed. If you need any more hints, just let me know.

Cheers

Alex