I'm Professor Seth Frey and today we're going to learn about collections of things in Python. Today we've learned about lots of different kinds of things in Python. The idea of true and false. We've learned how to do a bunch of things with text represented as strings of characters. We've learned to change and compare and otherwise manipulate lots of other things. From that foundation, we're going to build up a little further. We're going to learn how to make lists of things. We're going to learn how to access those lists to pull the things out of them, our little collections. We're going to learn to change and grow list to transform list. We're also going to see that strings are lists. We're going to learn to generalize the concepts of things that make a list a list and apply those to strings. We see a lot of parallels between the strings and lists. We're going to start by running our boilerplate code, our little code block. Then we'll get into what a list is. The syntax for list is, use a square bracket. Look around for your keyboard on where the square bracket is, and then you put an item in the list and you separate all the items in the list by commas. This list which is being assigned to the variable, my sublist has three elements. It has the string a, has a string b, and has the string c. If we run this, we'll see that my list's value is a list of three elements, a, b, and c. What's the type of my_list? We check its type and it's this new type of thing. It's a list, it's a type of collection. We can put other things in list. We can put numbers in list. There are list they can contain duplicates. This list contains 1, 2, and 3, but a bunch of other ones as well. It has five elements, three of which are one. Lists can contain mix of things. This version of my_list, we overrode the old my_list, now have a new my_list. This my_list has the number 1, the string "1", the string "hi", the value True, and then the decimal or float number 100.5. It has all those things in it, it's a list of five things, it's five long. We can put other things in the list. This list has two commas, the string '4', and then the sum of 3 plus 5, whatever that is. Python's not going to keep that expression, it's going to process that expression and give us the output of it. Processing still happens in list. But once we get this primitive basic element that Python can't do anything more to, that's going to be the element of the list. The variables are where we store things and because we can store all things in a variable, we can store all variables in a list. This code block is defining the days of the month of lots of holidays that occur in December. Now we're going to define a variable called december_holidays composed of those numbers. When I print it out, december_holidays, I'm not going to get X-mas, and the beginning of hannukah, and end of hannukah, and newyears, and blackfriday, I'm going to get the numbers 25,10,18,31, and -3, because Black Friday is actually in November. We can have a list that has just one thing in it. This is a tiny list, that have a variable called list that is tiny. It's been assigned square bracket nine close bracket. There's no commas because there's only one thing, there's nothing to separate. When I print out list that's tiny, I'm going to get the same list back, and when I print the length of that list. Just like we could get the length of strings, we can get the length of a list. I'm going to get the Number 1, because it only has one thing in it. A list can also have no things in it. If I do open close square brackets, I have the empty list. The length of that list is zero. That doesn't seem like it would be useful right now. But zero, when you first learned about zero, you didn't think it'd be useful. Zero is the idea of nothing and the idea of nothing is valuable in other types of things. The empty list is going to be something that we go into a lot. Now that you have the basics of how to define a list and see its length. Try your luck, make sure you understand things and we'll move along.