Explanation for len(set())

Question

Quick question, I was coding a small program in Python and was looking for a way to make a condition to only allow the input of 4 numbers if that input has the same len that I want and to make sure that all the numbers in it are unique. After searching for a bit on the web I found several people giving the same solution, it was len(set(input)) == size (size of my number) in my case.

It works well, but I fail to understand how it really works and I want to know. I've also tried removing the len from that and it worked anyway so I have little clue what are the effects of each thing in this small piece of code.

So even though it allows me to create a condition that makes sure each of the 4 numbers are unique I would love it if someone could explain to me how it works.

Thanks in advance.

PS: If anyone wonders what it is I'm practicing by making my own version of Cows & Bulls. So given a random number using random.sample the user then has to try to guess what the number is. The conditions I mentioned were used upon the user's input.

Answer 1

A set is a datastructure for:

constructing and manipulating unordered collections of unique elements.

These collections have to be hashable as well. But we are lucky: strings and characters are hashable.

The point is that if we construct a set form set(['1','4','1','5','2']), it will thus construct a collection of unique elements. So adding '1' twice here, does not makes any difference, the result is set(['1', '4', '5', '2']) (or more conveniently written {'1', '4', '5', '2'}). With len(..) we obtain the size of a collection. So the size of that set is the number of unique characters of the input.

So if we write len(set(some_string)) we will first turn a string into a set. This is possible since a string is an iterable of the characters. So Python sees a string as an ordered collection of characters. 'abc', is a collection of 'a', 'b', and 'c'. So every digit is seen as an element and added to the set.

But adding the same digit a second time, has no effect. So that means that we end up with a set where every digit is added once. By then calculating the len(..) we obtain the number of unique digits. We can then compare that number with a given number.