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Filter

Name: Filter
Uploaded: 2015-08-31T13:30:12-04:00
Duration: 309 s
Description: Filters are a very useful utility in programming, letting you refine a group of items into just the ones that meet your criteria. It's a "must be this tall to ride" sign for your data!

5:09 with Kenneth Love

Filters are a very useful utility in programming, letting you refine a group of items into just the ones that meet your criteria. It's a "must be this tall to ride" sign for your data!

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filter() takes a function and an iterable. The function, like with map(), takes only a single argument and is applied to each item in the iterable. If the function returns a truthy value for the item, the item is sent back to filter() which, ultimately, returns a new iterable of the filtered items.

You can achieve the same effect with [item for item in iterable if func(item)]. Again, like with map(), this can be more quickly readable for small applications.

I mentioned filterfalse(). This function works just like filter() but only returns things where the filter function gives back a False or non-truthy value. You can read more in the documentation.

Watch our comprehensions workshop if you want more information.

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I'll be honest, out of all the things that we have and 0:00

are going to talk about in this course filter is probably the one I use the most. 0:03

Filter, like map, takes an iterable and a function and 0:08

runs that function against each item in the iterable. 0:11

If the function returns a truthy value for 0:14

the item then that item gets added to the iterable that filter returns. 0:16

While it's quite a simple concept, I think you'll find that it's very handy. 0:19

So filter is one of the things that I probably use the most out of 0:24

all of Functional Programming. 0:28

I find it really super handy. 0:30

So, let's talk about it. 0:32

I mean I already gave you a bit of an introduction of it, so 0:33

I don't wanna talk about it too much, but the idea is we have an iterable, 0:37

we have a function, we run that function on each item in the iterable, and 0:41

if that function returns true for that item, then we keep that item around and 0:45

we throw away the ones where that function returns false. 0:51

So that means, we have to write a function again. 0:54

So let's write this new function here, we'll call it is_long_book and 0:57

it's gonna take a book. 1:01

Often you'll find that map functions would end up being something like 1:03

apply sales price, and filter functions start with like 1:09

is or are or something like that, where you're kind of like asking a question. 1:15

This naming convention just kind of helps with like oh yeah, 1:19

I'm filtering this to where it is a long book and then I'm gonna do whatever. 1:21

That's kind of the idea. 1:25

So let's just say here Does a book have 600 or more pages? 1:26

That's how we're defining a long book, we're saying, hey 600 pages. 1:35

So return book.number_of_pages >= 600. 1:38

So if a book has 600 pages or more, then we're gonna return true. 1:43

If a book has 599 pages or less, we're gonna return false. 1:50

That's all we're doing. 1:54

Filters are not always super simple functions, 1:55

you might have a complex function, but they always end up returning just, true or 1:59

false, or they return an object. 2:03

They return something truthy. 2:05

So, let's say long_books = 2:06

filter(is_long_book, BOOKS), 2:10

and just like with map, if we wanna be able to get to each of these items, 2:16

we need to put this into a list. 2:19

So, let's print len books, and 2:22

let's print len long books. 2:27

All right. 2:31

So there are 28 books total, and 12 of them are long books. 2:34

Stephen King definitely likes to write a lot of books, 2:37

or he likes to write a lot of words and thus a lot of pages in his books. 2:42

So what we've done, just to recap here, we're running a filter. 2:46

So is_long_book takes each item that's in BOOKS and runs this filter over it. 2:49

So pretty simple, pretty straightforward. 2:56

Again, this might be one where you're like, hey, 2:59

that looks an awful lot like something you did in that comprehensions workshop. 3:02

And I'd be like, yep, you caught me, it is. 3:06

So let's see how to do this as a comprehension. 3:10

So we'll say long_books2, and 3:12

we'll say book for book in BOOKS if, and 3:16

then here we could say is_long_book(book), 3:21

or we can just do if book.number_of_pages is >= 600. 3:27

So we can eliminate the use of this function all together. 3:34

And if we print long_books2 then we still get those same numbers, 3:38

so list comprehensions we get kind of two ideas. 3:44

We can apply a function at the beginning, and that gives us the map, or 3:49

we can do an if condition at the end, and that gives us a filter. 3:53

You can of course combine those, and we're gonna talk about that in a minute. 3:56

So, the idea though is you can do these two different ways. 4:00

Just like with map, which do you pick, do you pick filter, or 4:04

do you pick a list comprehension? 4:07

Again it really kinda comes down to, how hardcore this code going to end up being. 4:09

Is it going to be that I'm filtering this down one time? 4:15

Sure, use that list comp especially if it's a simple easy filter. 4:18

Is it gonna be hey, I've got to be able to call the same 4:22

kind of filter on a whole bunch of different things? 4:25

Or I need to be able to use this filter at a bunch of random points, or 4:28

is it that I need to be able to filter down this large list of items, 4:31

then I also need to be able to filter individual small items occasionally, 4:34

then filter and the filter function probably make more sense. 4:39

So it kinda becomes one of those things where you just learn it as you go. 4:42

If you want to get back an iterable of items that are falsey for 4:46

the function, you can change the inside of your filter function so 4:49

that it returns false instead of true for positive matches. 4:52

This can be kind of confusing logic though, so in the Itertools module 4:55

there's another function named filter false that works just like filter, but 4:59

Functional Python

Filter

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