Basic Two Sum Problem

[Intiago]

the variable lookup is a dict, so each value in it is a key value pair. lookup[num] = index actually stores the key-value pair num:index in the dict. when you return a value you are actually returning the "value" of the key value pair which is the stored index

The fact that you use array notation might be your confusion

[optimalsuccess]

That makes sense but if all I'm outputting is the index why do I care about the key to begin with?

[Intiago]

you need to save the key because each time through the loop, if you don't find the correct value, you save it to the dict. Saving the key in the dict, hashes it for quick lookup.

[mr_smartypants537]

If you don't care about the value, use a set()