Partials

Lambdas let us write throwaway functions. 0:00 Sometimes, though, 0:03 we need to be able to build up arguments to a function over time. 0:03 Maybe you have a function that sorts a variable based on another 0:07 function that you pass in, but you want 0:10 to offer some quick access functions with the sorting function already set. 0:12 This is where a fairly new part of the functools library comes in handy. 0:16 A little function named partial. 0:20 Partial lets us create a function with an incomplete or partial list of arguments. 0:23 This idea of having partial arguments to a function 0:28 also leads to another functional programming aspect known as currying. 0:31 Currying lets us return alternate versions of our function 0:35 from inside of the function based on the number of arguments that come in. 0:38 Let's go see about partials. 0:42 >> Now, I have to admit that partials are something 0:45 I don't use a whole lot in Python and neither is currying. 0:47 But that said, just because I don't use them all the time, 0:50 doesn't mean they're not handy and good to know about. 0:53 And it also doesn't mean that you won't find good uses for them. 0:55 And I feel it's better to give you more tools to use than I necessarily use myself 0:58 because you might find them really, really useful later on. 1:03 So let's talk about partials. 1:06 Partials let us partially call or partially apply a function. 1:08 The idea is if you think about a function that takes more than one argument, right? 1:14 We'll say a function takes two arguments. 1:17 If we can always supply one of those arguments, 1:20 then we only have to supply the second one whenever we want to use that function. 1:23 I think it's easier just to illustrate this with a function. 1:26 So let's do a function, we'll say mark-down. 1:29 So we're gonna take a book, and we're gonna take a discount. 1:31 And we're gonna, of course, copy the book. 1:36 And then we're gonna say the book's price should be equal to the rounded 1:38 version of the book's price minus the book's price times the discount. 1:43 And we want two decimal points on that. 1:49 And then we're gonna return the book. 1:51 Okay, so nothing really fancy here. 1:55 This is a lot like our sales price function from earlier, only in this case 1:57 we're taking a customizable discount, so you can discount the book by 0.2 for 20%. 2:01 You can mark it down by 0.5, for half price, stuff like that. 2:06 In fact, let's do that. 2:10 Let's say our standard mark down is partial mark down. 2:13 Mark down is the function that we want to call, so partial, 2:19 we're gonna import this here in a second, partial lets us partially apply the thing. 2:23 And then this is the thing we want to partially apply, mark down. 2:27 And the argument that we want to set is we wanna set discount and 2:31 we wanna set it to 0.2. 2:35 Okay. 2:37 So let's come up here and we need to import partial from func tools. 2:38 Okay. So the standard partial. 2:45 So now let's do print(standard(BOOKS[0]).price). 2:47 That should show us the markdown 2:56 price of that book. 3:02 Yeah, we get 10.84. 3:07 And if we were to do books, say 5, we get 6.39. 3:08 Okay, so there's our markdown price. 3:14 So that's cool. 3:16 So let's do another one here. 3:17 Let's do a half price. 3:18 So half = partial(mark_down, discount=.5) so 3:19 if we change this to half instead of standard now we get $4. 3:26 So that's all great, that's all well and good, but 3:35 here's where this comes in really handy. 3:38 So let's say that we want to mark down to half price all the books. 3:41 So we would map half to BOOKS. 3:50 Or maybe we wanna do it only to our long books, right? 3:55 That was our function earlier, right? 4:04 is_long_book Is long book, yeah. 4:06 All right so our long books, we're gonna make half price. 4:09 So then let's just print a list of half price books. 4:13 And there we go. 4:22 These books here, whatever these books are, 4:23 these are the long books, and we're gonna mark them off half price. 4:25 We're gonna mark them to half of their normal price. 4:30 So that's pretty cool! 4:33 We were able to combine a map and filter and our partial. 4:34 So we can write all of these partials for all of our common discounts, and 4:38 then we're just like, oh yeah, let's apply 50% off to these ten books, or 4:42 75% off to these books or whatever. 4:46 So that's where partials are. 4:49 Partials are handy for that kind of stuff. 4:49