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

Posted by Bozhong Tao
Bozhong Tao
18,365 Points

covers({"Python"}) would return ["Python Basics"] would not work for this code, however it worked in my atom console@@

I get the "Bummer! Didn't get the right output from covers." prompt using the follow code:

code_failed_test.py 
def covers(arg):
    subject = []
    for key, value in COURSES.items():
        if value & arg == arg:
            subject.append(key)
    return subject

If I replace the content in the if statement(replace "if value & arg == arg:" into "if value & arg:"), I can pass the first task, however I can print out the correct output in my atom console using the code block above.

code_passed_test.py 
def covers(arg):
    subject = []
    for key, value in COURSES.items():
        if value & arg:
            subject.append(key)
    return subject

output = covers({"Python"})
print(output)

I wonder why is that? is there anything specific I should be aware of when comparing intersection result using '==' or it's a bug?

Thanks for checking out!

3 Answers

Bill Leddy
Bill Leddy
2,720 Points
Bill Leddy
Bill Leddy
2,720 Points

The & intersection operator returns a set (or None). If the set returned is only a partial match for arg your append does not execute.

Bozhong Tao
Bozhong Tao
18,365 Points
Bozhong Tao
Bozhong Tao
18,365 Points

Thanks for the info Bill :) However, the code block above can print out ["Python Basics"] in my atom console, however it will fail the task check unless I replace the if statement into 

if value & arg:

So why "if value & arg:" work and "if value & arg == arg" would fail?

Bill Leddy
Bill Leddy
2,720 Points
Bill Leddy
Bill Leddy
2,720 Points

I think what’s going on is that in the case of an intersection (&) only some of the items in the args set may be returned. If there are 3 items in args the intersection may return only 1 or 2 of them. That still means that you want the course in your final output but the 2 items in the returned set will not exactly match args.

BTW I solved it ( eventually) with a differen approach but your solution was cleaner.

Bozhong Tao
Bozhong Tao
18,365 Points
Bozhong Tao
Bozhong Tao
18,365 Points

Hey Bill, thanks again for the response :) That make sense, I assumed the arg used by the task checker would only contain 1 item(In fact it could contain many) . that explains why the "if value & arg == arg" would fail. If the arg = {"Python", "CSS"} then the if statement would be False. Cheers!

PS.. What is your 'different approach'? Am interested :)