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Sometimes, an entire function, even an anonymous one, is just too much. Partial functions let us partially apply a function. Partials aren't as common as the other things we've covered, but they're still a handy tool.
functools.partial lets you preset some arguments to a function. You can then call the new function with the remaining arguments as needed. This often ends up being really handy when used with map() and filter().
Lambdas let us write throwaway functions.
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Sometimes, though,
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we need to be able to build up
arguments to a function over time.
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Maybe you have a function that
sorts a variable based on another
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function that you pass in, but you want
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to offer some quick access functions
with the sorting function already set.
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This is where a fairly new part of
the functools library comes in handy.
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A little function named partial.
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Partial lets us create a function with an
incomplete or partial list of arguments.
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This idea of having partial
arguments to a function
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also leads to another functional
programming aspect known as currying.
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Currying lets us return alternate
versions of our function
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from inside of the function based on
the number of arguments that come in.
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Let's go see about partials.
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>> Now, I have to admit
that partials are something
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I don't use a whole lot in Python and
neither is currying.
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But that said, just because I
don't use them all the time,
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doesn't mean they're not handy and
good to know about.
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And it also doesn't mean that you
won't find good uses for them.
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And I feel it's better to give you more
tools to use than I necessarily use myself
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because you might find them really,
really useful later on.
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So let's talk about partials.
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Partials let us partially call or
partially apply a function.
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The idea is if you think about a function
that takes more than one argument, right?
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We'll say a function takes two arguments.
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If we can always supply
one of those arguments,
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then we only have to supply the second one
whenever we want to use that function.
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I think it's easier just to
illustrate this with a function.
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So let's do a function,
we'll say mark-down.
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So we're gonna take a book, and
we're gonna take a discount.
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And we're gonna, of course, copy the book.
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And then we're gonna say the book's
price should be equal to the rounded
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version of the book's price minus
the book's price times the discount.
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And we want two decimal points on that.
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And then we're gonna return the book.
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Okay, so nothing really fancy here.
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This is a lot like our sales price
function from earlier, only in this case
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we're taking a customizable discount, so
you can discount the book by 0.2 for 20%.
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You can mark it down by 0.5, for
half price, stuff like that.
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In fact, let's do that.
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Let's say our standard mark
down is partial mark down.
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Mark down is the function that
we want to call, so partial,
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we're gonna import this here in a second,
partial lets us partially apply the thing.
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And then this is the thing we want
to partially apply, mark down.
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And the argument that we want to
set is we wanna set discount and
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we wanna set it to 0.2.
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Okay.
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So let's come up here and
we need to import partial from func tools.
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Okay.
So the standard partial.
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So now let's do
print(standard(BOOKS[0]).price).
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That should show us the markdown
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price of that book.
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Yeah, we get 10.84.
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And if we were to do books,
say 5, we get 6.39.
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Okay, so there's our markdown price.
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So that's cool.
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So let's do another one here.
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Let's do a half price.
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So half = partial(mark_down,
discount=.5) so
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if we change this to half instead
of standard now we get $4.
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So that's all great,
that's all well and good, but
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here's where this comes in really handy.
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So let's say that we want to mark
down to half price all the books.
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So we would map half to BOOKS.
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Or maybe we wanna do it only
to our long books, right?
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That was our function earlier, right?
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is_long_book Is long book, yeah.
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All right so our long books,
we're gonna make half price.
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So then let's just print
a list of half price books.
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And there we go.
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These books here,
whatever these books are,
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these are the long books, and
we're gonna mark them off half price.
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We're gonna mark them to
half of their normal price.
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So that's pretty cool!
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We were able to combine a map and
filter and our partial.
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So we can write all of these partials for
all of our common discounts, and
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then we're just like, oh yeah,
let's apply 50% off to these ten books, or
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75% off to these books or whatever.
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So that's where partials are.
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Partials are handy for that kind of stuff.
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