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Python 3, an illustrated tour Transcripts

Chapter: Language syntax

Lecture: Walk-through: Other Changes

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0:01
In this video we're going to look at other test.py.

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Open it up in your editor, let's run it and make sure that it runs.

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Okay, it looks like there's one failure,

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that's because there's one test function, cool.

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Let's go through it. Lazy range, get the 100th item

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from the vals variable, store it in the variable named hun.

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So we have a vals variable here

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and it's range from 42 up to but not including two million

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jumping by 32 and we want the 100th item here,

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let's try and see if we can slice it off

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hun is equal to vals 100, let's run that and see if it works.

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Okay, it looks like that worked,

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so the range function even though it's lazy in Python 3,

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allows you to do some slicing off of it,

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let's do a map, find the 100th item from mapping fn,

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which is this function right up here, to vals using the map function

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store the result in hun function.

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So we say map we're going to map a function fn to vals.

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And we want the hundredth guy from that.

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So hun_fn is equal to that and let's see if we can slice off the hundredth guy.

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We'll run it and we get an error, map is not subscriptable.

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So even though range is lazy, map is also lazy,

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but map doesn't support this index operation,

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so what we would need to do to get the hundredth item

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is do something like this,

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seq is equal to the map of that,

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and then for i in range 100 hun_fn equals next seq

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let's see if that works

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and I have a typo here let's change that and run it again.

2:09
Okay, and so it looks like that works

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so this just shows you something that you may need to do

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in this case I need to jump through a little hoop

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and call next 100 times to get this item from my sequence here

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because it's lazy.

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Now there are pluses and minuses to this,

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in Python 2 map materialized the list for you and you can slice a list

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but if you have an arbitrary long list it might take a bunch of memory

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so there are tradeoffs depending on what behavior

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also I could course this into a list and do that

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but again, we're going from 42 to 2 million

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so this is pretty big I might not want to do that.

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Sorting, sort the nums list as if they were integers

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store the result in ordered, hint look at the sorted and the keys parameter

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look at sorted and the keys parameter,

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so there's a built in function called sorted and it has a keys parameter.

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Okay, so in Python 2, Python 2 would allow us to sort lists with arbitrary types

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and a Python 3 wants to be a little bit more explicit.

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So if we just say ordered = sorted nums, let's run that and see what happens

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I get a type error, less than operation is not supported

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between instances of string and int

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that's because I've got a string in here and I've also got integers in here.

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So it wants me to sort these as if they were integers

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so one thing we can do is we can cast them to integers

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so we could make a little for loop or do a list comprehension and cast them to integers.

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But they key parameter in the sorted function

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will allow us to apply an arbitrary function to an item that needs to be sorted

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and will sort based on that.

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And this gives us the original ordered will now give us back the original list

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but sorted as if they were integers.

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So let's run and see if it works.

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Okay, it looks like that worked.

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So this is sorting that list as if they were integers.

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Name leakage, sum the square of the numbers in nums,

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store the result in square sum

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so I'm going to put them right here in this space

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and note that I've got some variables here that are just floating around

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that maybe someone created or maybe I created if I'm typing code

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and if I want to square a bunch of numbers and I've got them in sequence,

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one way to do that is to use a list comprehension,

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I can say nums is equal to, or I've already got nums,

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so maybe sq is equal to num squared for num in nums.

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And then I want to sum that,

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and so I'm going to say sq.sum is equal to the sum of this whole guy here.

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Let's run that see if it works.

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Okay, I got an error here, unsupported operand for ** a string and an int,

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again I've got this string in here, so maybe I want to corse these to ints.

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Let's corse them to ints, and then do that see if that works.

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Okay, it looks like it worked this might just seem like a silly thing

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but you'll note that in this case I used num

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as my variable in my list comprehension here

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and in Python 2 if I used num here when I get out of this list comprehension

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the value of no will be the last value of this list comprehension.

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In Python 3 that behavior is changed and there is no "leakage"

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of this variable into the surrounding scope here

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so num stays at 42.

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So I put an X in here and a num in here

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in case you used those in a list comprehension.

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Now if you did these in another way, if you made a for loop and used num in there

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then you would overwrite this num guy

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so it's just something to be aware of if you're not familiar with that

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in Python 2 on list comprehensions,

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note that generator expressions and set and dictionary comprehensions

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behave as in Python 3, there's no leakage there

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but in the list comprehension there is.

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So this video showed some of the things that changed in Python 3,

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we have laziness as a general theme

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so range and map are both lazy

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they support somewhat different interface so be aware of that.

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Sorting, when you sort different types, you need to be specific

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and make sure that they support sorting

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and so we can use the key parameter of the sorted function to help enable that

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and there's no name leakage in list comprehensions now.