How do I properly initialize a set in Python?

Question

I initially defined a set using destinations={"x",} . Then I tried to remove something using destinations=destinations.discard("x") . However, the terminal says AttributeError: 'NoneType' object has no attribute 'discard' when I try to run it. It seems that it is not yet a set. I included a comma with the braces when initializing it and at least it should be a string. What am I doing wrong?

Answer 1

Quamrana's comment answers the essence of your question, but to break it down:

destinations={"x","y",}   #"y" added for the sake of demonstration
destinations = destinations.discard("x")
print(destinations)       # Output = '' i.e. no output

If you see type(destinations) , it will output as <class 'NoneType'>

That's because discard (similar to many other methods) returns a NoneType object. (The same is true for other methods like list.append(). However, note that is this not true for all methods. For example dict.pop(key) returns a non-NoneType Object.)

Now for the proper way to do it:

destinations={"x","y",}   #"y" added for the sake of demonstration
destinations.discard("x")
print(destinations)       # Output: {'y'}

If you see type(destinations) , it will output as <class 'set'>

Interestingly, if you were to reassign the variable, your original set would still be affected.

destinations = {"x","y"}
new_set = destinations
new_set.discard('x')
print(new_set)         # output = {'y'}
print(destinations)    # output = {'y'}

Since you're learning CS50, i think David will cover this at some point when teaching lists: Have fun learning :)